libiconv-full: fix compile-time linking error GCC7

libiconv-full: fix compile-time linking error GCC7

LEDE Flyspray Task 1091:
Fix libiconv-full 'undefined reference' compile linker error using GCC7 Musl
Tested with targets x86 (i386 and x86_64)
Addition of CFLAGS "std=gnu89" fixes the linker issues, credit to harrylwc
Issue found with 'minidlna' package, which depends on 'libiconv-full'
Error in compile log:
../lib/.libs/libiconv.so: undefined reference to `aliases_lookup'
../lib/.libs/libiconv.so: undefined reference to `aliases2_lookup'
collect2: error: ld returned 1 exit status
Makefile:64: recipe for target 'iconv_no_i18n' failed

Signed-off-by: Jake Staehle <jacob@staehle.us>