[gdbserver] Move malloc.h include to server.h.

[gdbserver] Move malloc.h include to server.h.

This patch moves all includes of malloc.h, which were introduced
purely to get access to alloca's declaration, to server.h, next
to the include of alloca.h.

There is one exception: gdbreplay.c, which does not include server.h.
In this case, the include of alloca.h was simply moved up a bit, next
to the include of malloc.h.

gdb/gdbserver/ChangeLog:

        * gdbreplay.c: Move include of alloca.h up, next to include of
        malloc.h.
        * server.h: Add include of malloc.h.
        * mem-break.c: Remove include of malloc.h.
        * server.c, tracepoint.c, utils.c, win32-low.c: Likewise.