build: don't .gitignore m4/ax_pthread.m4

It was reported that the addition of that file without adding an
exception to m4/.gitignore breaks the tarball autotools build because
the file isn't distributed.

Fixes: #2862
PR-URL: #2885
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>

m4/.gitignore

@@ -1,4 +1,5 @@
 # Ignore libtoolize-generated files.
 *.m4
 !as_case.m4
+!ax_pthread.m4
 !libuv-check-flags.m4