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/sections/cper-section-nvidia.h b/sections/cper-section-nvidia.h
index 973c3af..b05fdde 100644
--- a/sections/cper-section-nvidia.h
+++ b/sections/cper-section-nvidia.h
@@ -1,10 +1,18 @@
 #ifndef CPER_SECTION_NVIDIA_H
 #define CPER_SECTION_NVIDIA_H
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #include <json.h>
 #include "../edk/Cper.h"
 
 json_object *cper_section_nvidia_to_ir(void *section);
 void ir_section_nvidia_to_cper(json_object *section, FILE *out);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif