tools: build: Fixup host c flags

Commit 0c3b7e42 ("tools build: Add support for host programs format")
introduced host_c_flags which referenced CHOSTFLAGS. The actual name of the
variable is HOSTCFLAGS. Fix this up.

Fixes: 0c3b7e42

 ("tools build: Add support for host programs format")
Signed-off-by: Laura Abbott <labbott@redhat.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>