imx: hab: Fix usage of packed attribute

commit ed286bc80e ("imx: hab: Check if CSF is valid before authenticating
image") makes use of "__packed" as a prefix to the "struct hab_hdr"
declaration.

With my compiler "gcc version 7.2.1 20171011 (Linaro GCC 7.2-2017.11)" we
get:

./arch/arm/include/asm/mach-imx/hab.h:42:25: error: expected ‘=’, ‘,’, ‘;’,
‘asm’ or ‘__attribute__’ before ‘{’ token
 struct __packed hab_hdr {

Fix this problem by including <linux/compiler.h>

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Cc: Utkarsh Gupta <utkarsh.gupta@nxp.com>
Cc: Breno Lima <breno.lima@nxp.com>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>

arch/arm/include/asm/mach-imx/hab.h

@@ -9,6 +9,7 @@
 #define __SECURE_MX6Q_H__
 
 #include <linux/types.h>
+#include <linux/compiler.h>
 
 /*
  * IVT header definitions