common: use char array for version string

Use a char array instead of a pointer, which allows using the version
variables in a constant context.

common/common.h

@@ -65,8 +65,8 @@ enum {
     DATA_EOF    = -2,
 };
 
-extern const char *const mpv_version;
-extern const char *const mpv_builddate;
+extern const char mpv_version[];
+extern const char mpv_builddate[];
 
 char *mp_format_time(double time, bool fractions);
 char *mp_format_time_fmt(const char *fmt, double time);

common/version.c

@@ -22,5 +22,5 @@
 #define BUILDDATE "UNKNOWN"
 #endif
 
-const char *const mpv_version  = "mpv " VERSION;
-const char *const mpv_builddate = BUILDDATE;
+const char mpv_version[]  = "mpv " VERSION;
+const char mpv_builddate[] = BUILDDATE;