Ensure __always_inline is defined on operating systems that don't have it

For example, Alpine Linux fails to build libosmocore without us locally
defining __always_inline.

Change-Id: I0c22aac410e668f8fb8d5de57281bcaeb343eeb1
diff --git a/src/conv_acc_sse_impl.h b/src/conv_acc_sse_impl.h
index e6eaaa6..560af62 100644
--- a/src/conv_acc_sse_impl.h
+++ b/src/conv_acc_sse_impl.h
@@ -22,6 +22,12 @@
  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
 
+/* Some distributions (notably Alpine Linux) for some strange reason
+ * don't have this #define */
+#ifndef __always_inline
+#define __always_inline         inline __attribute__((always_inline))
+#endif
+
 extern int sse41_supported;
 
 /* Octo-Viterbi butterfly