Make Parser::seen volatile

Marlin/gcode.h

@@ -120,7 +120,7 @@ public:
 
     // Code seen bit was set. If not found, value_ptr is unchanged.
     // This allows "if (seen('A')||seen('B'))" to use the last-found value.
-    static bool seen(const char c) {
+    static volatile bool seen(const char c) {
       const uint8_t ind = c - 'A';
       if (ind >= COUNT(param)) return false; // Only A-Z
       const bool b = TEST(codebits[ind >> 3], ind & 0x7);
@@ -132,7 +132,7 @@ public:
 
     // Code is found in the string. If not found, value_ptr is unchanged.
     // This allows "if (seen('A')||seen('B'))" to use the last-found value.
-    static bool seen(const char c) {
+    static volatile bool seen(const char c) {
       const char *p = strchr(command_args, c);
       const bool b = !!p;
       if (b) value_ptr = DECIMAL_SIGNED(p[1]) ? &p[1] : (char*)NULL;