linux_spi: add missing include

Some defines (e.g. _IOC_SIZEBITS) are defined in linux/ioctl.h,
so it must be included before it is used, by SPI_IOC_MESSAGE
from linux/spi/spidev.h

Fixes build errors with the musl C library, as seen in these
Buildroot autobuilder failures:

    http://autobuild.buildroot.org/results/2a3/2a3744007c630c267575a638ebcd83a4b97644f5/build-end.log
    http://autobuild.buildroot.org/results/3de/3de936d9be79e151e66af15193084d82a0f2c04a/build-end.log

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

diff -durN flashrom-0.9.8.orig/linux_spi.c flashrom-0.9.8/linux_spi.c
--- flashrom-0.9.8.orig/linux_spi.c	2015-10-28 19:42:38.480285847 +0100
+++ flashrom-0.9.8/linux_spi.c	2015-10-28 19:43:15.492994613 +0100
@@ -27,6 +27,7 @@
 #include <ctype.h>
 #include <unistd.h>
 #include <linux/types.h>
+#include <linux/ioctl.h>
 #include <linux/spi/spidev.h>
 #include <sys/ioctl.h>
 #include "flash.h"