libfdt: Fix redefined uintptr_t warning for USE_HOSTCC

Compiling U-Boot in an old OS environment (RedHat-7.3  :-)  gives the
following warnings from FDT:

include/libfdt_env.h:50: warning: redefinition of 'uintptr_t'
/usr/include/stdint.h:129: warning: 'uintptr_t' previously declared here

Fix: Protect the definition of uintptr_t when compiling on the host
system.

Signed-off-by: Gerald Van Baren <vanbaren@cideas.com>