build-sys: include errno.h instead of argp.h

build-sys: include errno.h instead of argp.h

configure should include errno.h instead of argp.h when
checking for presence of program_invocation_short_name
uclibc defines this to be const char* unlike util-linux-ng
which defines this to be char* so this error goes unnoticed
on glibc/eglibc systems.

here is the error it fixes

in file included from mountP.h:14:0,
                 from cache.c:29:
/home/kraj/work/slugos/build/tmp-slugos-uclibc/sysroots/nslu2le/usr/include/errno.h:55:46: error: conflicting types for '__progname'
../../../include/c.h:118:14: note: previous declaration of '__progname' was here
make[3]: *** [cache.lo] Error 1

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Jonathan Liu <net147@gmail.com>