GCC: Disable the -Wmissing-field-initializers in GCC 4

Perfectly valid C++11 code trying to default-initialize an object with
{} is being warned. GCC 5 and up only warn if you initialize some fields
and not others.

qcborvalue.h:68:25: error: missing initializer for member 'QCborError::c' [-Werror=missing-field-initializers]
     QCborError error = {};
                         ^

Task-number: QTBUG-68889
Change-Id: I6efb28c3145047559ec0fffd1538577de250e283
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Liang Qi <liang.qi@qt.io>

mkspecs/features/qt_common.prf

@@ -81,6 +81,8 @@ clang {
     }
 } else: gcc:!intel_icc {
     QMAKE_CXXFLAGS_WARN_ON += -Wvla
+    # GCC 5 fixed -Wmissing-field-initializers for when there are no initializers
+    lessThan(QT_GCC_MAJOR_VERSION, 5): QMAKE_CXXFLAGS_WARN_ON += -Wno-missing-field-initializers
     # GCC 5 introduced -Wdate-time
     greaterThan(QT_GCC_MAJOR_VERSION, 4): QMAKE_CXXFLAGS_WARN_ON += -Wdate-time
     # GCC 6 introduced these