Makefile: fix "output" target name

Without the right target name, make will rebuild the target (dwmblocks)
every time make is run, even if the source files haven't changed.

Makefile

@@ -2,13 +2,13 @@
 
 PREFIX = /usr/local
 
-output: dwmblocks.o
+dwmblocks: dwmblocks.o
 	gcc dwmblocks.o -lX11 -o dwmblocks
 dwmblocks.o: dwmblocks.c config.h
 	gcc -c -lX11 dwmblocks.c
 clean:
 	rm -f *.o *.gch dwmblocks
-install: output
+install: dwmblocks
 	mkdir -p $(DESTDIR)$(PREFIX)/bin
 	cp -f dwmblocks $(DESTDIR)$(PREFIX)/bin
 	chmod 755 $(DESTDIR)$(PREFIX)/bin/dwmblocks