Fix a build issue for a test

This commit adds #ifdef __cpluscplus guard in odintrin.h to enable
the build of related test in C++.

Change-Id: I89ddbdd56b10657ac7e4c71e2f26ed791550b493

vp10/common/odintrin.h

@@ -6,6 +6,10 @@
 #include "aom_dsp/vpx_dsp_common.h"
 #include "aom_ports/bitops.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /*Smallest blocks are 4x4*/
 # define OD_LOG_BSIZE0 (2)
 /*There are 5 block sizes total (4x4, 8x8, 16x16, 32x32 and 64x64).*/
@@ -44,4 +48,8 @@ extern uint32_t OD_DIVU_SMALL_CONSTS[OD_DIVU_DMAX][2];
   We define a special version of the macro to use when x can be zero.*/
 # define OD_ILOG(x) ((x) ? OD_ILOG_NZ(x) : 0)
 
+#ifdef __cplusplus
+}  // extern "C"
+#endif
+
 #endif