Makefile: Use '--ignore-errors unused' with lcov

We are a bit overzealous in specifying arguments to 'lcov -r', listing
files (via wildcard) that are not actually in the tracefile.

This is harmless, but will cause newer lcov to generate an error message
of type 'unused'. Reduce this error to a warning.

Signed-off-by: Reza Arbab <arbab@linux.ibm.com>

Makefile.main

@@ -413,7 +413,7 @@ skiboot.info: coverage external/pflash/pflash.info external/gard/gard.info
 	lcov -q -r $@ 'external/gard/*' -o $@
 	lcov -q -a $@ -a external/pflash/pflash.info -o $@
 	lcov -q -a $@ -a external/gard/gard.info -o $@
-	lcov -q -r $@ $(LCOV_EXCLUDE) -o $@ --rc lcov_branch_coverage=1
+	lcov -q -r $@ $(LCOV_EXCLUDE) -o $@ --rc lcov_branch_coverage=1 --ignore-errors unused
 
 doc:
 	make -C doc html