Fix compilation with Clang on MacOS.

Clang does not like methods to be declared static inline,
and then have their definition somewhere else. The error message was:

./qdoublevector2d_p.h:186:49: error: conflicting types for 'dotProduct'
Q_DECL_CONSTEXPR inline double QDoubleVector2D::dotProduct(const QDoubleVector2D &v1, const QDoubleVector2D &v2)
                                                ^
./qdoublevector2d_p.h:101:43: note: previous declaration is here
    Q_DECL_CONSTEXPR static inline double dotProduct(const QDoubleVector2D &v1, const QDoubleVector2D &v2);
                                          ^
In file included from qgeocircle.cpp:49:
./qdoublevector3d_p.h:234:49: error: conflicting types for 'dotProduct'
Q_DECL_CONSTEXPR inline double QDoubleVector3D::dotProduct(const QDoubleVector3D &v1, const QDoubleVector3D &v2)
^
./qdoublevector3d_p.h:104:43: note: previous declaration is here
    Q_DECL_CONSTEXPR static inline double dotProduct(const QDoubleVector3D &v1, const QDoubleVector3D &v2);
                                              ^
./qdoublevector3d_p.h:239:58: error: conflicting types for 'crossProduct'
Q_DECL_CONSTEXPR inline QDoubleVector3D QDoubleVector3D::crossProduct(const QDoubleVector3D &v1, const QDoubleVector3D &v2)
         ^
./qdoublevector3d_p.h:105:52: note: previous declaration is here
    Q_DECL_CONSTEXPR static inline QDoubleVector3D crossProduct(const QDoubleVector3D &v1, const QDoubleVector3D &v2);

Change-Id: Ica9305f628c393e3d37e19097ad7343c03e68ef7
Reviewed-by: Sergio Ahumada <sahumada@blackberry.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>