Fix missing "struct iovec" definition on some x86-linux.

Fix missing "struct iovec" definition on some x86-linux.

The following patch...

    commit 3116063bd617de56fbc3bad046a692b1fb363a9d
    Date:   Fri Jun 27 09:52:29 2014 +0100
    Subject: Tidy #include lists

... introduced a build failure on certain x86 GNU/Linux distributions
(reproduced on SuSE 10 and RHES4) due to "struct iovec" not being
defined. This struct is defined in <sys/uio.h>, which used to be
explicitly included, but no longer is after the commit above was
applied.

    [...]/i386-linux-nat.c: In function 'fetch_xstateregs':
    [...]/i386-linux-nat.c:325:16: error: storage size of 'iov' isn't known
    [...]/i386-linux-nat.c: In function 'store_xstateregs':
    [...]/i386-linux-nat.c:348:16: error: storage size of 'iov' isn't known
    make[2]: *** [i386-linux-nat.o] Error 1

It seems to be working on newer GNU/Linux distros thanks to indirect
inclusion of <sys/uio.h>, but it does not work on some other versions
of the same distros. This is why indirect includes of public APIs
should be avoided if at all possible.

This patch fixes the issue by adding the explicit include back.

gdb/ChangeLog:

        * i386-linux-nat.c, x86-linux-nat.c: Add <sys/uio.h> #include.