Makefile: eliminate malloc.c warning

Makefile: eliminate malloc.c warning

malloc.c: In function ‘sys_trim’:
malloc.c:4289:20: warning: unused variable ‘newsize’ [-Wunused-variable]
malloc.c: In function ‘destroy_mspace’:
malloc.c:5452:13: warning: unused variable ‘base’ [-Wunused-variable]