[U-Boot] build mkimage warning

[Bin Meng]

Hi Tom, Simon,

With latest main branch, I see some warnings when building mkimage:

  HOSTLD  tools/mkimage
LDFLAGS="" python ./lib/libfdt/setup.py \
                "-Wall -Wstrict-prototypes -O2 -fomit-frame-pointer
-include ./include/libfdt_env.h -idirafterinclude
-idirafter./arch/x86/include -I./lib/libfdt -I./tools
-DCONFIG_SYS_TEXT_BASE=0xfff00000 -DUSE_HOSTCC -D__KERNEL_STRICT_NAMES
-D_GNU_SOURCE " lib/libfdt/fdt.c lib/libfdt/fdt_ro.c
lib/libfdt/fdt_rw.c lib/libfdt/fdt_strerror.c lib/libfdt/fdt_wip.c
lib/libfdt/fdt_region.c lib/libfdt/fdt_sw.c tools/libfdt_wrap.c
In file included from /usr/local/include/python2.7/Python.h:8,
                 from tools/libfdt_wrap.c:125:
/usr/local/include/python2.7/pyconfig.h:1193:1: warning:
"_POSIX_C_SOURCE" redefined
In file included from /usr/include/stdint.h:26,
                 from ././include/compiler.h:19,
                 from ././include/libfdt_env.h:12,
                 from <command-line>:0:
/usr/include/features.h:162:1: warning: this is the location of the
previous definition
In file included from /usr/local/include/python2.7/Python.h:8,
                 from tools/libfdt_wrap.c:125:
/usr/local/include/python2.7/pyconfig.h:1215:1: warning:
"_XOPEN_SOURCE" redefined
In file included from /usr/include/stdint.h:26,
                 from ././include/compiler.h:19,
                 from ././include/libfdt_env.h:12,
                 from <command-line>:0:
/usr/include/features.h:164:1: warning: this is the location of the
previous definition
mv _libfdt.so tools/_libfdt.so

Regards,
Bin