Update libX11 .so version to 6.4.0

There have been API additions since the previous release, so the
.so version needs to change.
Signed-off-by: Keith Packard <keithp@keithp.com>

src/Makefile.am

@@ -364,7 +364,7 @@ if XKB
 USE_XKB_LIBS = $(XKB_LIBS)
 endif
 
-libX11_la_LDFLAGS = -version-number 6:3:0 -no-undefined
+libX11_la_LDFLAGS = -version-number 6:4:0 -no-undefined
 
 libX11_la_LIBADD = \
 	$(LTLIBOBJS) \