Fix MIPS gdbserver build

Since 3955f0de839d4bc65b4ff0605d3033f6f0168ece which correctly places
$STDLIBS after conftest.c, gdb/gdbserver/configure can discover
pread64() (not exposed in NDK header) in MIPS's libc.a and define
HAVE_PREAD64.

Add prototype for pread64() otherwise -Werror may fail compilation

Change-Id: I47f475d25daa67ebfd9789983e1698d51f09aaf4