devel/libvhdi: Fix build when NLS option is disabled

The libvhdi autoconf macros add -I/usr/local/include to AM_CPPFLAGS as a side
effect of processing --with-libintl-prefix=/usr/local.
When NLS is enabled, the port passes that argument and the global include path
is set correctly, making <fuse/fuse.h> resolvable.
When NLS is disabled, --disable-nls is passed instead, the include path is
never set, and the FUSE compilation fails.

PR: 295440
Approved by: antoine
Sponsored by: Netflix