Remove redundant include

It's also included later, guarded by support for ECC keys, and actually
that's the only case where we need it.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/library/pkparse.c b/library/pkparse.c
index e1422df..687c2c0 100644
--- a/library/pkparse.c
+++ b/library/pkparse.c
@@ -26,7 +26,6 @@
 #include "mbedtls/oid.h"
 #include "mbedtls/platform_util.h"
 #include "mbedtls/error.h"
-#include "pk_internal.h"
 
 #include <string.h>