Fix another -Os strcoll build issue.

While there are now clean -Os build and test results on x86_64 (given
my patch <https://sourceware.org/ml/libc-alpha/2018-02/msg00602.html>,
pending review), testing with -Os with build-many-glibcs.py shows the
build is still failing with -Os everywhere except for x86_64, x86 and
s390x.

There are a variety of different build failures, but the most common
seem to be in strcoll / wcscoll, similar to existing such cases where
DIAG_* are used to disable -Wmaybe-uninitialized.  There are various
different failures even within those functions.  This patch fixes one
particular case that seems quite common, where the warning appears at
the declarations of seq1 and seq2.

Tested with build-many-glibcs.py that this fixes the -Os build for
aarch64-linux-gnu with GCC 7.

	* string/strcoll_l.c: Include <libc-diag.h>.
	(STRCOLL): Ignore -Wmaybe-uninitialized for -Os around
	declarations of seq1 and seq2.