[libc-trusty] Fix prototype conflicts in malloc.c

Inline brk because the prototype is non-standard.

Make sbrk non-static to match the prototype. We could rename it to
trusty_sbrk and keep the prototype static, but eventually we should
un-hack the preprocessor include of dlmalloc.c, and once we do that
we'll need a non-static sbrk.

Bug: 110161494
Change-Id: I2bedceb83fd7abcece917423a43705eb0464259a
1 file changed