libnm: avoid duplicate typedefs for NMClient/NMDevice

clang 3.4.2-9.el7 does not like this:

  $ clang -DHAVE_CONFIG_H -I. -I..  -I../src/libnm-core-public -I./src/libnm-core-public -I../src/libnm-client-public -I./src/libnm-client-public -pthread -I/usr/include/gio-unix-2.0/ -I/usr/include/glib-2.0 -I/usr/lib64/glib-2.0/include   -DGLIB_VERSION_MIN_REQUIRED=GLIB_VERSION_2_40 -DGLIB_VERSION_MAX_ALLOWED=GLIB_VERSION_2_40  -Wall -Werror -Wextra -Wdeclaration-after-statement -Wfloat-equal -Wformat-nonliteral -Wformat-security -Wimplicit-function-declaration -Winit-self -Wmissing-declarations -Wmissing-include-dirs -Wmissing-prototypes -Wpointer-arith -Wshadow -Wstrict-prototypes -Wundef -Wvla -Wno-duplicate-decl-specifier -Wno-format-y2k -Wno-missing-field-initializers -Wno-sign-compare -Wno-tautological-constant-out-of-range-compare -Wno-unknown-pragmas -Wno-unused-parameter  -Qunused-arguments -Wunknown-warning-option -Wtypedef-redefinition -Warray-bounds -Wparentheses-equality -Wunused-value -Wimplicit-fallthrough  -fno-strict-aliasing -fdata-sections -ffunction-sections -Wl,--gc-sections -g -O2 -MT examples/C/glib/examples_C_glib_add_connection_libnm-add-connection-libnm.o -MD -MP -MF examples/C/glib/.deps/examples_C_glib_add_connection_libnm-add-connection-libnm.Tpo -c -o examples/C/glib/examples_C_glib_add_connection_libnm-add-connection-libnm.o `test -f 'examples/C/glib/add-connection-libnm.c' || echo '../'`examples/C/glib/add-connection-libnm.c
  ...
  ../src/libnm-client-public/nm-client.h:149:31: error: redefinition of typedef 'NMClient' is a C11 feature [-Werror,-Wtypedef-redefinition]
  typedef struct _NMClient      NMClient;
                                ^

Our code base is C11 internally (actually "-std=gnu11"), but this problem
happens when we build the example. The warning is actually correct, because
our public headers should be more liberal (and possibly be C99 or even C89,
this is undefined).

Fixes: 649314dd ('libnm: replace nm-types.h by defining the types in respective headers')
59 jobs for th/fix-libnm-includes in 45 minutes and 59 seconds (queued for 4 seconds)
Name Stage Failure
failed
t_debian:10 Test
Build type: native build
$ mv /tmp/nm-test.log .
$ exit $r
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