fix build warning with arm64 gcc 5.4
GCC 5.4 reports below warning on Arm64
```
warning: 'vst1q_u8' is static but used in inline function 'mbedtls_xor' which is not static
```
This inline function miss `static`, others have the keyword
Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/library/common.h b/library/common.h
index c20f6b2..ec30a7d 100644
--- a/library/common.h
+++ b/library/common.h
@@ -165,7 +165,10 @@
* \param b Pointer to input (buffer of at least \p n bytes)
* \param n Number of bytes to process.
*/
-inline void mbedtls_xor(unsigned char *r, const unsigned char *a, const unsigned char *b, size_t n)
+static inline void mbedtls_xor(unsigned char *r,
+ const unsigned char *a,
+ const unsigned char *b,
+ size_t n)
{
size_t i = 0;
#if defined(MBEDTLS_EFFICIENT_UNALIGNED_ACCESS)