Use extern C guards in all headers

The project uses a mix of C and C++ requiring
extern "C" guards from C++ code both within the
project & from C++ apps that use libcper. That
won't be required with this change.

Change-Id: I835dd05166732ca213c72eae2904815a8769599b
Signed-off-by: Karthik Rajagopalan <krajagopalan@nvidia.com>
diff --git a/cper-utils.h b/cper-utils.h
index 22280fe..e4c14a1 100644
--- a/cper-utils.h
+++ b/cper-utils.h
@@ -4,6 +4,10 @@
 #define GUID_STRING_LENGTH 48
 #define TIMESTAMP_LENGTH   24
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #include "common-utils.h"
 #include <json.h>
 
@@ -42,4 +46,8 @@
 //The available severity types for CPER.
 extern const char *CPER_SEVERITY_TYPES[4];
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif