QMake: fix GCC 9 -Wdeprecated-copy warnings

Not visible in QMake, because of too old C++ standard used to compile it,
but in the qttools copy. Fix here, as the authorative source, first.

Change-Id: I2552eccfaab2cef0863686dcd888f2a5f25ca29f
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

qmake/library/proitems.h

@@ -68,6 +68,7 @@ class ProString {
 public:
     ProString();
     ProString(const ProString &other);
+    ProString &operator=(const ProString &) = default;
     PROITEM_EXPLICIT ProString(const QString &str);
     PROITEM_EXPLICIT ProString(const QStringRef &str);
     PROITEM_EXPLICIT ProString(const char *str);

qmake/library/qmakeparser.h

@@ -111,7 +111,6 @@ private:
 
     struct BlockScope {
         BlockScope() : start(nullptr), braceLevel(0), special(false), inBranch(false), nest(NestNone) {}
-        BlockScope(const BlockScope &other) { *this = other; }
         ushort *start; // Where this block started; store length here
         int braceLevel; // Nesting of braces in scope
         bool special; // Single-line conditionals inside loops, etc. cannot have else branches