Tests: Update OSS-Fuzz Makefile.

All .c files can be built as separate fuzz targets. This simplifies
the Makefile by allowing us to use wildcards instead of having a
Makefile target for each fuzz target.

tests/ossfuzz/Makefile

@@ -1,7 +1,12 @@
-fuzz: fuzz.c
-	$(CC) $(CFLAGS) -c fuzz.c -I ../../src/liblzma/api/
-	$(CXX) $(CXXFLAGS) $(LIB_FUZZING_ENGINE) fuzz.o -o $(OUT)/fuzz \
-	      ../../src/liblzma/.libs/liblzma.a
+FUZZ_TARGET_SRCS = $(wildcard *.c)
+FUZZ_TARGET_BINS = $(FUZZ_TARGET_SRCS:.c=)
+
+all: $(FUZZ_TARGET_BINS)
+
+%: %.c
+	$(CC) $(CFLAGS) -c $< -I ../../src/liblzma/api/ ;
+	$(CXX) $(CXXFLAGS) $(LIB_FUZZING_ENGINE) $(<:.c=.o) -o $(OUT)/$@ \
+		../../src/liblzma/.libs/liblzma.a ;
 
 clean:
 	rm -f *.o