slim: fix build with gcc>=11

srcpkgs/slim/patches/slim-1.3.6-gcc11.patch

@@ -0,0 +1,12 @@
+https://bugs.gentoo.org/786498
+--- a/panel.cpp
++++ b/panel.cpp
+@@ -48,7 +48,7 @@
+ 		gcm = GCGraphicsExposures;
+ 		gcv.graphics_exposures = False;
+ 		WinGC = XCreateGC(Dpy, Win, gcm, &gcv);
+-		if (WinGC < 0) {
++		if (WinGC == 0) {
+ 			cerr << APPNAME
+ 				<< ": failed to create pixmap\n.";
+ 			exit(ERR_EXIT);