fix partial-build test

sometimes, the scope difference is solely determined by the list of source files,
not by the flags.

tests/libzstd_partial_builds.sh

@@ -22,7 +22,6 @@ mustBeAbsent() {
 }
 
 # default compilation : all features enabled
-make clean > /dev/null
 $ECHO "testing default library compilation"
 CFLAGS= make -C $DIR/../lib libzstd.a > $INTOVOID
 nm $DIR/../lib/libzstd.a | $GREP "\.o" > tmplog