isl: add isl_priv.h to the sources list

Otherwise it will be missing from the release tarball.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/intel/isl/Makefile.sources

 	isl_gen4.h \
 	isl_gen6.c \
 	isl_gen6.h \
+	isl_priv.h \
 	isl_storage_image.c
 
 ISL_GEN7_FILES = \