Fixed common misspellings.

Mostly found by 'codespell'.

src/libstrongswan/plugins/pkcs11/pkcs11_library.c

@@ -495,7 +495,7 @@ typedef struct {
 	CK_SESSION_HANDLE session;
 	/* pkcs11 library */
 	pkcs11_library_t *lib;
-	/* attributes to retreive */
+	/* attributes to retrieve */
 	CK_ATTRIBUTE_PTR attr;
 	/* number of attributes */
 	CK_ULONG count;

src/libstrongswan/plugins/pkcs11/pkcs11_manager.h

@@ -32,7 +32,7 @@ typedef struct pkcs11_manager_t pkcs11_manager_t;
  *
  * @param data		user supplied data, as passed to pkcs11_manager_create()
  * @param p11		loaded PKCS#11 library token belongs to
- * @param slot		slot number the event occured in
+ * @param slot		slot number the event occurred in
  * @param add		TRUE if token was added to the slot, FALSE if removed
  */
 typedef void (*pkcs11_manager_token_event_t)(void *data, pkcs11_library_t *p11,