build: remove static libraries before running ar

If a library rule is being built, one of the files it contains was
rebuilt, so we need to regenerate the whole library anyway.

Remove the previous copy of the library to make sure nothing is
left from the previous build.

Change-Id: I835f721d5bf7cc908c343f47de78bd1f44be58b8
Signed-off-by: Daniel Verkamp <daniel.verkamp@intel.com>

mk/spdk.common.mk

@@ -140,6 +140,7 @@ LINK_CXX=\
 # Archive $(OBJS) into $@ (.a)
 LIB_C=\
 	$(Q)echo "  LIB $(notdir $@)"; \
+	rm -f $@; \
 	ar crDs $@ $(OBJS)
 
 # Clean up generated files listed as arguments plus a default list