fix regression in make clean

commit 40005d3ff6b5674184c296e47e5dfff9a46a0be0 renamed the build
directories from build-* to build/*. update make clean to remove the
new build tree.

Makefile

@@ -51,7 +51,7 @@ SRC_DIRS = gcc-$(GCC_VER) binutils-$(BINUTILS_VER) musl-$(MUSL_VER) \
 all:
 
 clean:
-	rm -rf gcc-* binutils-* musl-* gmp-* mpc-* mpfr-* isl-* build-* linux-*
+	rm -rf gcc-* binutils-* musl-* gmp-* mpc-* mpfr-* isl-* build build-* linux-*
 
 distclean: clean
 	rm -rf sources