build-sys: do not make qemu-ga link with pixman

Since commit d52c454aad ("contrib: add
vhost-user-gpu"), qemu-ga is linking with pixman.

This is because the Make-based build-system use a global namespace for
variables, and we rely on "main.o-libs" for different linking targets.

Note: this kind of variable clashing is hard to fix or prevent
currently.  meson should help, as declarations have a linear
dependency and doesn't rely so much on variables and clever tricks.

Note2: we have a lot of main.c (or other duplicated names!) in
tree. Imho, it would be annoying and a bad workaroud to rename all
those to avoid conflicts like I did here.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1811670

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20200311160923.882474-1-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

contrib/vhost-user-gpu/Makefile.objs

@@ -1,7 +1,7 @@
-vhost-user-gpu-obj-y = main.o virgl.o vugbm.o
+vhost-user-gpu-obj-y = vhost-user-gpu.o virgl.o vugbm.o
 
-main.o-cflags := $(PIXMAN_CFLAGS) $(GBM_CFLAGS)
-main.o-libs := $(PIXMAN_LIBS)
+vhost-user-gpu.o-cflags := $(PIXMAN_CFLAGS) $(GBM_CFLAGS)
+vhost-user-gpu.o-libs := $(PIXMAN_LIBS)
 
 virgl.o-cflags := $(VIRGL_CFLAGS) $(GBM_CFLAGS)
 virgl.o-libs := $(VIRGL_LIBS)