LibC: Support X modifier for scanf

This was currently crashing Half-Life because it was a considered an
"Unknown" specifier. We can use the same case statement as the regular
hex format conversion (lower case 'x'), as the backend
to convert the number already supports upper/lower case input, hence
we get it for free :^)

Userland/Libraries/LibC/scanf.cpp

@@ -488,6 +488,7 @@ extern "C" int vsscanf(const char* input, const char* format, va_list ap)
                 conversion_specifier = ConversionSpecifier::Unsigned;
                 break;
             case 'x':
+            case 'X':
                 format_lexer.consume();
                 conversion_specifier = ConversionSpecifier::Hex;
                 break;