fix 'licenses' -> 'license' in several places

pkgs/applications/video/mjpg-streamer/default.nix

···
     homepage = http://sourceforge.net/projects/mjpg-streamer/;
     description = "MJPG-streamer takes JPGs from Linux-UVC compatible webcams, filesystem or other input plugins and streams them as M-JPEG via HTTP to webbrowsers, VLC and other software";
     platforms = platforms.linux;
-    licenses = licenses.gpl2;
+    license = licenses.gpl2;
   };
 }

pkgs/data/fonts/corefonts/default.nix

···
     homepage = "http://corefonts.sourceforge.net/";
     description = "Microsoft's TrueType core fonts for the Web";
     platforms = platforms.all;
-    licenses = licenses.unfreeRedistributable;
+    license = licenses.unfreeRedistributable;
     # Set a non-zero priority to allow easy overriding of the
     # fontconfig configuration files.
     priority = 5;

pkgs/development/tools/analysis/garcosim/tracefilesim/default.nix

···
     description = "Ease the analysis of existing memory management techniques, as well as the prototyping of new memory management techniques.";
     homepage = "https://github.com/GarCoSim";
     maintainers = [ maintainers.cmcdragonkai ];
-    licenses = licenses.gpl2;
+    license = licenses.gpl2;
     platforms = platforms.linux;
   };