pop3: Fix POP3_TYPE_ANY signed compilation warning

POP3_TYPE_ANY, or ~0, is written to pop3c->preftype in lib/pop3c.c, an
unsigned int variable.  The result of ~0 is -1, which caused a warning
due to writing a negative number to an unsigned variable.  To fix this,
make the expression ~0U so that its value is considered the unsigned
number UINT_MAX which is what SASL_AUTH_ANY does in curl_sasl.h.

lib/pop3.h

@@ -93,7 +93,7 @@ extern const struct Curl_handler Curl_handler_pop3s;
 
 /* Authentication type values */
 #define POP3_TYPE_NONE      0
-#define POP3_TYPE_ANY       ~0
+#define POP3_TYPE_ANY       ~0U
 
 /* This is the 5-bytes End-Of-Body marker for POP3 */
 #define POP3_EOB "\x0d\x0a\x2e\x0d\x0a"