LIB: Move tfm_vprintf definition to private header

This function should not be accessed outside of the tfm_log and
tfm_log_unpriv libraries and therefore does not need to be exposed in
the public header. Add a new private header which is then included by
the logging source files.

Change-Id: Ic6ca5d4ad753b6e6abaedaf27d0b17b9ebe6f1b3
Signed-off-by: Jackson Cooper-Driver <jackson.cooper-driver@arm.com>
diff --git a/lib/tfm_vprintf/src/tfm_vprintf.c b/lib/tfm_vprintf/src/tfm_vprintf.c
index 7878fb8..5999dab 100644
--- a/lib/tfm_vprintf/src/tfm_vprintf.c
+++ b/lib/tfm_vprintf/src/tfm_vprintf.c
@@ -14,6 +14,7 @@
 #include <stddef.h>
 
 #include "tfm_vprintf.h"
+#include "tfm_vprintf_priv.h"
 
 #define LOG_RAW_VALUE UINT8_C(60)