multimedia/ffnvcodec-headers: explicitly set PREFIX

Without this, upstream's Makefile uses /usr/local, which may be
incorrect on systems with unusual PREFIX.

PR:	272105
Approved by: Alex S. (for multimedia@)

multimedia/ffnvcodec-headers/Makefile

@@ -15,6 +15,7 @@ USE_GITHUB=	yes
 GH_ACCOUNT=	FFmpeg
 GH_PROJECT=	nv-codec-headers
 
+MAKE_ARGS+=	PREFIX="${PREFIX}"
 NO_ARCH=	yes
 
 .include <bsd.port.mk>