rgbfix: improve Makefile

Make object filename explicit and add "clean" target.

Signed-off-by: Vegard Nossum <vegard.nossum@gmail.com>

src/rgbfix/Makefile

@@ -1,8 +1,13 @@
+obj := \
+	main.o
+
 all: rgbfix
 
+clean:
+	rm -rf rgbfix $(obj)
+
+rgbfix: $(obj)
+	gcc -Wall -o $@ $^
+
 .c.o:
 	gcc -Wall -I. -I.. -Iinclude -g -c -o $@ $<
-
-rgbfix: $(patsubst %.c,%.o,$(wildcard *.c))
-	gcc -Wall -o $@ $^
-