build: Fix build failure on mingw with --disable-shared --enable-relocatable.
Reported by Michele Locati <michele@locati.it>
in <https://lists.gnu.org/archive/html/bug-gettext/2019-04/msg00052.html>.
* gettext-runtime/intl/Makefile.am (AM_CPPFLAGS): Map 'relocate2' to a different
symbol.
