inttypes: avoid autoconf warning

inttypes: avoid autoconf warning

I noticed this while updating libvirt to the latest gnulib.

configure.ac:80: warning: AC_REQUIRE: `gl_STDINT_H' was expanded before it was required
gnulib/m4/inttypes.m4:18: gl_INTTYPES_INCOMPLETE is expanded from...
gnulib/m4/inttypes.m4:12: gl_INTTYPES_H is expanded from...
gnulib/m4/gnulib-comp.m4:379: gl_INIT is expanded from...
configure.ac:80: the top level

* m4/inttypes.m4 (gl_INTTYPES_INCOMPLETE): Only expand once.
* m4/stdint.m4 (gl_STDINT_H): Likewise.

Signed-off-by: Eric Blake <eblake@redhat.com>