error_prints: fix potential program_invocation_name type conflict

error_prints: fix potential program_invocation_name type conflict

uClibc-ng declares program_invocation_name as const.
This causes the build failure below:

error_prints.c:40:14: error: conflicting types for ‘program_invocation_name’
 extern char *program_invocation_name;
              ^~~~~~~~~~~~~~~~~~~~~~~
In file included from error_prints.c:32:0:
.../output/host/x86_64-buildroot-linux-uclibc/sysroot/usr/include/errno.h:54:20: note: previous declaration of ‘program_invocation_name’ was here
 extern const char *program_invocation_name, *program_invocation_short_name;
                    ^~~~~~~~~~~~~~~~~~~~~~~

* error_prints.c (program_invocation_name): Declare only in case
of [!HAVE_PROGRAM_INVOCATION_NAME].