Rename cplusplus files for consistency

Change-Id: I908ea337ffb7229a33af3364b5374209c7e9df54
diff --git a/drivers/mailbox/CMakeLists.txt b/drivers/mailbox/CMakeLists.txt
index bd2dae3..9e56d37 100644
--- a/drivers/mailbox/CMakeLists.txt
+++ b/drivers/mailbox/CMakeLists.txt
@@ -18,4 +18,4 @@
 
 add_library(ethosu_mailbox STATIC)
 target_include_directories(ethosu_mailbox PUBLIC include)
-target_sources(ethosu_mailbox PRIVATE src/mailbox.cc)
+target_sources(ethosu_mailbox PRIVATE src/mailbox.cpp)