Write the CC rule such that it only tries to produce a single output file (the
.so). When including the .h files in the compiler invocation, this suggests we
should be producing pre-compiled headers too, which doesn't make sense in this
context, and Clang happens not to like.

Submitted a similar patch upstream:
https://lore.kernel.org/linux-wireless/20191105225751.1233-1-briannorris@chromium.org/T/#u
but this project is pretty much at the end of its life, as it's not needed for
kernels >=4.15.

--- a/Makefile
+++ b/Makefile
@@ -113,9 +113,9 @@ keys.c: utils/key2pub.py $(wildcard $(PUBKEY_DIR)/*.pem)
 	$(NQ) '  Trusted pubkeys:' $(wildcard $(PUBKEY_DIR)/*.pem)
 	$(Q)./utils/key2pub.py $(wildcard $(PUBKEY_DIR)/*.pem) $@
 
-$(LIBREG): regdb.h reglib.h reglib.c
+$(LIBREG): reglib.c regdb.h reglib.h
 	$(NQ) '  CC  ' $@
-	$(Q)$(CC) $(CFLAGS) $(CPPFLAGS) $(LDFLAGS) -o $@ -shared -Wl,-soname,$(LIBREG) $^ $(filter-out -lreg,$(LDLIBS))
+	$(Q)$(CC) $(CFLAGS) $(CPPFLAGS) $(LDFLAGS) -o $@ -shared -Wl,-soname,$(LIBREG) $< $(filter-out -lreg,$(LDLIBS))
 
 install-libreg-headers:
 	$(NQ) '  INSTALL  libreg-headers'