glib2: disable fortify source

The glib2 package fails to build when CONFIG_PKG_FORTIFY_SOURCE_1 or
CONFIG_PKG_FORTIFY_SOURCE_2 is enabled in the OpenWrt config:

In file included from ../glib/libcharset/localcharset.c:28:
/home/stijn/Development/OpenWrt/openwrt/staging_dir/toolchain-powerpc64_e5500_gcc-11.2.0_musl/include/fortify/stdio.h: In function 'snprintf':
/home/stijn/Development/OpenWrt/openwrt/staging_dir/toolchain-powerpc64_e5500_gcc-11.2.0_musl/include/fortify/stdio.h:101:9: error: format not a string literal, argument types not checked [-Werror=format-nonliteral]
  101 |         return __orig_snprintf(__s, __n, __f, __builtin_va_arg_pack());
      |         ^~~~~~
/home/stijn/Development/OpenWrt/openwrt/staging_dir/toolchain-powerpc64_e5500_gcc-11.2.0_musl/include/fortify/stdio.h: In function 'sprintf':
/home/stijn/Development/OpenWrt/openwrt/staging_dir/toolchain-powerpc64_e5500_gcc-11.2.0_musl/include/fortify/stdio.h:110:17: error: format not a string literal, argument types not checked [-Werror=format-nonliteral]
  110 |                 __r = __orig_snprintf(__s, __b, __f, __builtin_va_arg_pack());
      |                 ^~~
/home/stijn/Development/OpenWrt/openwrt/staging_dir/toolchain-powerpc64_e5500_gcc-11.2.0_musl/include/fortify/stdio.h:114:17: error: format not a string literal, argument types not checked [-Werror=format-nonliteral]
  114 |                 __r = __orig_sprintf(__s, __f, __builtin_va_arg_pack());
      |                 ^~~

Disable fortify source for the package as a workaround.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>
(cherry picked from commit 723c4360)