intel: wrap intel_bufmgr.h C code for C++ compilation/linking

We need this include in porting changes for the OpenGL ES
conformance suite.

v2: remove c_plusplus usage

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>

intel/intel_bufmgr.h

@@ -38,6 +38,10 @@
 #include <stdint.h>
 #include <stdio.h>
 
+#if defined(__cplusplus)
+extern "C" {
+#endif
+
 struct drm_clip_rect;
 
 typedef struct _drm_intel_bufmgr drm_intel_bufmgr;
@@ -308,4 +312,8 @@ int drm_intel_get_eu_total(int fd, unsigned int *eu_total);
 
 /** @{ */
 
+#if defined(__cplusplus)
+}
+#endif
+
 #endif /* INTEL_BUFMGR_H */