[linux] Avoid unused-but-set variable warning in gcc 4.6

Temporary modification to prevent valgrind.h from breaking compilation
with gcc 4.6.  When this problem is fixed upstream, a new and
unmodified copy of valgrind.h should be imported.

Signed-off-by: Thomas Miletich <thomas.miletich@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/x86/include/valgrind/valgrind.h

    Ignored if addr == 0.
 */
 #define VALGRIND_MALLOCLIKE_BLOCK(addr, sizeB, rzB, is_zeroed)    \
-   {unsigned int _qzz_res;                                        \
+   {unsigned int __unused _qzz_res;                               \
     VALGRIND_DO_CLIENT_REQUEST(_qzz_res, 0,                       \
                                VG_USERREQ__MALLOCLIKE_BLOCK,      \
                                addr, sizeB, rzB, is_zeroed, 0);   \
    Ignored if addr == 0.
 */
 #define VALGRIND_FREELIKE_BLOCK(addr, rzB)                        \
-   {unsigned int _qzz_res;                                        \
+   {unsigned int __unused _qzz_res;                               \
     VALGRIND_DO_CLIENT_REQUEST(_qzz_res, 0,                       \
                                VG_USERREQ__FREELIKE_BLOCK,        \
                                addr, rzB, 0, 0, 0);               \