gitignore: Ignore shader generated files

As of d98bc0b6 there are two files that are automatically generated:
ui/shader/texture-blit-frag.h and /ui/shader/texture-blit-vert.h. None
of them is wanted to be tracked by git. Put them into the ignore file
then.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

.gitignore

@@ -17,6 +17,8 @@
 /trace/generated-tcg-tracers.h
 /trace/generated-ust-provider.h
 /trace/generated-ust.c
+/ui/shader/texture-blit-frag.h
+/ui/shader/texture-blit-vert.h
 /libcacard/trace/generated-tracers.c
 *-timestamp
 /*-softmmu