Ignore all foreign m4 files

Now that the dpkg m4 files are namespaced, we can ignore all foreign
ones safely.

m4/.gitignore

@@ -1,33 +1,2 @@
-codeset.m4
-gettext.m4
-glibc21.m4
-glibc2.m4
-iconv.m4
-intdiv0.m4
-intl.m4
-intldir.m4
-intmax.m4
-inttypes_h.m4
-inttypes.m4
-inttypes-pri.m4
-isc-posix.m4
-lcmessage.m4
-lib-ld.m4
-lib-link.m4
-lib-prefix.m4
-lock.m4
-longdouble.m4
-longlong.m4
-nls.m4
-po.m4
-printf-posix.m4
-progtest.m4
-signed.m4
-size_max.m4
-stdint_h.m4
-uintmax_t.m4
-ulonglong.m4
-visibility.m4
-wchar_t.m4
-wint_t.m4
-xsize.m4
+*.m4
+!dpkg-*.m4