libs/dtc: fix build on x86_64

libs/dtc: fix build on x86_64

Fixes build error:
```
x86_64-openwrt-linux-musl-gcc -L/home/sandu/work/lede/staging_dir/target-x86_64_musl-1.1.15/usr/lib -L/home/sandu/work/lede/staging_dir/target-x86_64_musl-1.1.15/lib -L/home/sandu/work/lede/staging_dir/toolchain-x86_64_gcc-5.4.0_musl-1.1.15/usr/lib -L/home/sandu/work/lede/staging_dir/toolchain-x86_64_gcc-5.4.0_musl-1.1.15/lib -znow -zrelro  -fPIC -shared -Wl,--version-script=libfdt/version.lds -Wl,-soname,libfdt.so.1 -o libfdt/libfdt-1.4.2.so libfdt/fdt.o libfdt/fdt_ro.o libfdt/fdt_wip.o libfdt/fdt_sw.o libfdt/fdt_rw.o libfdt/fdt_strerror.o libfdt/fdt_empty_tree.o libfdt/fdt_addresses.o
/home/sandu/work/lede/staging_dir/toolchain-x86_64_gcc-5.4.0_musl-1.1.15/lib64/gcc/x86_64-openwrt-linux-musl/5.4.0/../../../../x86_64-openwrt-linux-musl/bin/ld: libfdt/fdt.o: relocation R_X86_64_32S against `.rodata' can not be used when making a shared object; recompile with -fPIC
libfdt/fdt.o: error adding symbols: Bad value
collect2: error: ld returned 1 exit status
Makefile:283: recipe for target 'libfdt/libfdt-1.4.2.so' failed
make[4]: *** [libfdt/libfdt-1.4.2.so] Error 1
```

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>