libobs: Add API function to get version string

Some plugins are using the OBS_VERSION macro obs-config.h to specify the
current version, which will bake the string in to the plugin, making it
so that if the plugin is not replaced for a patch, it could potentially
have the incorrect version.  This makes it so that a plugin/frontend can
get the current version string that's baked in to libobs itself.

libobs/obs.c

@@ -884,6 +884,11 @@ uint32_t obs_get_version(void)
 	return LIBOBS_API_VER;
 }
 
+const char *obs_get_version_string(void)
+{
+	return OBS_VERSION;
+}
+
 void obs_set_locale(const char *locale)
 {
 	struct obs_module *module;