Fix compile warnings about always_inline functions in GCC 5.2.0

The warning is "always_inline function might not be inlinable", fixed by
adding inline keyword. See:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55830

JerryScript-DCO-1.0-Signed-off-by: Geoff Gustafson geoff@linux.intel.com

jerry-core/jrt/jrt.c

@@ -25,7 +25,7 @@
  * @return true, if read was successful, i.e. offset + data_size doesn't exceed buffer size,
  *         false - otherwise.
  */
-bool __attr_always_inline___
+inline bool __attr_always_inline___
 jrt_read_from_buffer_by_offset (const uint8_t *buffer_p, /**< buffer */
                                 size_t buffer_size, /**< size of buffer */
                                 size_t *in_out_buffer_offset_p, /**< [in,out] offset to read from /
@@ -53,7 +53,7 @@ jrt_read_from_buffer_by_offset (const uint8_t *buffer_p, /**< buffer */
  * @return true, if write was successful, i.e. offset + data_size doesn't exceed buffer size,
  *         false - otherwise.
  */
-bool __attr_always_inline___
+inline bool __attr_always_inline___
 jrt_write_to_buffer_by_offset (uint8_t *buffer_p, /**< buffer */
                                size_t buffer_size, /**< size of buffer */
                                size_t *in_out_buffer_offset_p,  /**< [in,out] offset to read from /