[truetype] Fix `make multi'.

* src/truetype/ttgxvar.c: Include freetype/internal/services/svmetric.h
for the FT_Service_MetricsVariations type definition.

src/truetype/ttgxvar.c

@@ -45,6 +45,7 @@
 #include <freetype/internal/ftcalc.h>
 #include <freetype/internal/ftstream.h>
 #include <freetype/internal/sfnt.h>
+#include <freetype/internal/services/svmetric.h>
 #include <freetype/tttags.h>
 #include <freetype/ttnameid.h>
 #include <freetype/ftmm.h>