build: don't rebuild everything for libtest.la

We added a -fvisibilty=default to the libtest.la CFLAGS. When automake
sees that the CFLAGS are different from those of libxkbcommon.la, it
figures it should recompile all of the $(libxkbcommon_la_SOURCES),
instead of reusing the already-compiled object files.

But actually, the convenience library is not a shared object, it's just
used as an archive, thus the -fvisibility is entirely pointless. So
avoid the recompilations by just removing it.

Signed-off-by: Ran Benita <ran234@gmail.com>

Makefile.am

@@ -154,7 +154,6 @@ endif ENABLE_DOCS
 # Some tests need to use unexported symbols, so we link them against
 # a private copy of libxkbcommon with all symbols exposed.
 check_LTLIBRARIES = libtest.la
-libtest_la_CFLAGS = $(AM_CFLAGS) -fvisibility=default
 libtest_la_SOURCES = \
 	$(libxkbcommon_la_SOURCES) \
 	test/common.c \