Allow <vboot/crossystem.h> to be usable in C++ code.

BUG=None
TEST=build update_engine with a change that uses the header.
BRANCH=none

Change-Id: Icbfe9be615a4f7f4078a0a0cde64324908dea2a7
Reviewed-on: https://chromium-review.googlesource.com/174428
Commit-Queue: Richard Barnette <jrbarnette@chromium.org>
Tested-by: Richard Barnette <jrbarnette@chromium.org>
Reviewed-by: Bill Richardson <wfrichar@chromium.org>

host/include/crossystem.h

@@ -6,6 +6,10 @@
 #ifndef VBOOT_REFERENCE_CROSSYSTEM_H_
 #define VBOOT_REFERENCE_CROSSYSTEM_H_
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /* Reads a system property integer.
  *
  * Returns the property value, or -1 if error. */
@@ -28,4 +32,8 @@ int VbSetSystemPropertyInt(const char* name, int value);
  * Returns 0 if success, -1 if error. */
 int VbSetSystemPropertyString(const char* name, const char* value);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif  /* VBOOT_REFERENCE__CROSSYSTEM_H_ */