tpm: Add extern "C" declaration for C++ compatibility.

The mount-encrypted utility, which includes tlcl.h is in the process
of being converted to C++. To simplify inclusion in C++ code, add the
standard extern "C" linkage specifiers.

BRANCH=none
BUG=chromium:808303
TEST=compiles

Change-Id: I8a99a0e5cfcce64b27cad10735b08ba40434b222
Reviewed-on: https://chromium-review.googlesource.com/915345
Commit-Ready: Mattias Nissler <mnissler@chromium.org>
Tested-by: Mattias Nissler <mnissler@chromium.org>
Reviewed-by: Vadim Bendebury <vbendeb@chromium.org>
Reviewed-by: Andrey Pronin <apronin@chromium.org>

firmware/include/tlcl.h

@@ -15,6 +15,10 @@
 
 #include "tss_constants.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /*****************************************************************************/
 /* Functions implemented in tlcl.c */
 
@@ -226,4 +230,8 @@ uint32_t TlclGetVersion(uint32_t* vendor, uint64_t* firmware_version,
  */
 uint32_t TlclIFXFieldUpgradeInfo(TPM_IFX_FIELDUPGRADEINFO *info);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif  /* TPM_LITE_TLCL_H_ */