makefile: fix .so build rule

Use $^ vs $? to capture all prerequisites, rather than only newer ones.

Change-Id: I464ffb2913cf60f5ca5e07518ad4bf7da10baf87

build/make/Makefile

@@ -250,7 +250,7 @@ $(1):
 	$(qexec)$$(LD) -shared $$(LDFLAGS) \
             -Wl,--no-undefined -Wl,-soname,$$(SONAME) \
             -Wl,--version-script,$$(SO_VERSION_SCRIPT) -o $$@ \
-            $$(filter %.o,$$?) $$(extralibs)
+            $$(filter %.o,$$^) $$(extralibs)
 endef
 
 define lipo_lib_template