Parallel build fix: avoid multiple jobs in source/theme.

The top-level Makefile has three library targets which invoke submakes
in source/theme. Since the submake targets share object files, and make
isn't aware of dependencies between different make processes, a parallel
build can try to build the same object file more than once. This results
in failures like this, where two makes are trying to build the same
object:

Linking carlastyle.so
../../build/theme/Release/CarlaStyle.cpp.qt5.o: file not recognized: File truncated
collect2: error: ld returned 1 exit status

Work around this by having all the theme*.a targets depend on theme.

Makefile

@@ -112,13 +112,13 @@ $(MODULEDIR)/jackbridge.%.a: .FORCE
 $(MODULEDIR)/native-plugins.a: .FORCE
 	@$(MAKE) -C source/native-plugins
 
-$(MODULEDIR)/theme.a: .FORCE
+$(MODULEDIR)/theme.a: theme .FORCE
 	@$(MAKE) -C source/theme
 
-$(MODULEDIR)/theme.qt4.a: .FORCE
+$(MODULEDIR)/theme.qt4.a: theme .FORCE
 	@$(MAKE) -C source/theme qt4
 
-$(MODULEDIR)/theme.qt5.a: .FORCE
+$(MODULEDIR)/theme.qt5.a: theme .FORCE
 	@$(MAKE) -C source/theme qt5
 
 $(MODULEDIR)/%.posix32.a: .FORCE