BaseTools/header.makefile: remove "-c" from BUILD_CFLAGS

BaseTools/header.makefile: remove "-c" from BUILD_CFLAGS

Option "-c" is a mode selection flag (choosing between compiling and
linking); it should not be in BUILD_CFLAGS, which applies only to
compiling anyway. The compilation rule for C source files, in
"footer.makefile", already includes "-c" -- currently we have double "-c"
options.

This patch doesn't change behavior.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Ref: https://bugzilla.redhat.com/show_bug.cgi?id=1540244
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>