libnm-glib-aux: fix build error with GCC 15
Summary
Fix the following error with GCC 15:
../src/libnm-glib-aux/nm-shared-utils.c:35:42: error: initializer-string for array of 'char' is too long [-Werror=unterminated-string-initialization]
35 | const char _nm_hexchar_table_lower[16] = "0123456789abcdef";
| ^~~~~~~~~~~~~~~~~~
../src/libnm-glib-aux/nm-shared-utils.c:36:42: error: initializer-string for array of 'char' is too long [-Werror=unterminated-string-initialization]
36 | const char _nm_hexchar_table_upper[16] = "0123456789ABCDEF";
|