build: move copyright statement to a shared location

Now macosx_menubar.m and mpv.rc (win32) use the same copyright string.
(This is a bit roundabout, because mpv.rc can't use C constants. Also
the C code wants to avoid rebuilding real source files if only version.h
changed, so only version.c includes version.h.)

common/common.h

@@ -68,6 +68,7 @@ enum {
 
 extern const char mpv_version[];
 extern const char mpv_builddate[];
+extern const char mpv_copyright[];
 
 char *mp_format_time(double time, bool fractions);
 char *mp_format_time_fmt(const char *fmt, double time);

common/version.c

@@ -24,3 +24,4 @@
 
 const char mpv_version[]  = "mpv " VERSION;
 const char mpv_builddate[] = BUILDDATE;
+const char mpv_copyright[] = MPVCOPYRIGHT;