Update .gitignore

Don't ignore everything under build-aux

.gitignore

@@ -25,17 +25,24 @@ config.status
 libtool
 .deps/
 .dirstamp
-build-aux/
 *.lo
 *.o
 *~
 src/libsecp256k1-config.h
 src/libsecp256k1-config.h.in
 src/ecmult_static_context.h
-m4/libtool.m4
-m4/ltoptions.m4
-m4/ltsugar.m4
-m4/ltversion.m4
-m4/lt~obsolete.m4
+build-aux/config.guess
+build-aux/config.sub
+build-aux/depcomp
+build-aux/install-sh
+build-aux/ltmain.sh
+build-aux/m4/libtool.m4
+build-aux/m4/lt~obsolete.m4
+build-aux/m4/ltoptions.m4
+build-aux/m4/ltsugar.m4
+build-aux/m4/ltversion.m4
+build-aux/missing
+build-aux/compile
+build-aux/test-driver
 src/stamp-h1
 libsecp256k1.pc