build: add .metal to vpath to fix build error

Signed-off-by: Aman Karmani <aman@tmm1.net>

Makefile

@@ -13,6 +13,7 @@ vpath %.v    $(SRC_PATH)
 vpath %.texi $(SRC_PATH)
 vpath %.cu   $(SRC_PATH)
 vpath %.ptx  $(SRC_PATH)
+vpath %.metal $(SRC_PATH)
 vpath %/fate_config.sh.template $(SRC_PATH)
 
 TESTTOOLS   = audiogen videogen rotozoom tiny_psnr tiny_ssim base64 audiomatch