devel/gettext-tools: Fix build with non-default prefix

Building this port with a non-default prefix gives the following:

msgmerge.c:35:10: fatal error: 'textstyle.h' file not found
#include <textstyle.h>
         ^~~~~~~~~~~~~
1 error generated.
*** [msgmerge-msgmerge.o] Error code 1

That is, textstyle.h is present in the appropriate location under
our prefix, alas it's not found.

USES=localbase addresses this.

PR: 258286
Approved by: tijl (maintainer)