Fix -fno-common

diff -r c259e312716e vehicle.h
--- a/src/vehicle.h	Sat Oct 03 12:42:08 2020 +0000
+++ b/src/vehicle.h	Sat Oct 03 12:44:13 2020 +0000
@@ -70,7 +70,7 @@
 
   /* constants */
   float mu[128];		/* friction factor */
-} GrBreak;
+};
 
 struct _GrTireData {
   GrRef ref;