platform: add minimal Makefile.am for convenience

When working only with nm-platform and its testsuite, it's convenient to
call 'make', 'make check', etc from 'src/platform' directory.

configure.ac

@@ -687,6 +687,7 @@ src/settings/plugins/keyfile/tests/Makefile
 src/settings/plugins/keyfile/tests/keyfiles/Makefile
 src/settings/plugins/example/Makefile
 src/settings/tests/Makefile
+src/platform/Makefile
 src/platform/tests/Makefile
 src/devices/wimax/Makefile
 libnm-util/libnm-util.pc

src/platform/Makefile.am

@@ -0,0 +1 @@
+SUBDIRS = tests