Reshuffle inlined functions to fix MinGW-warnings.

When compiling QtQuick:

qv4value_p.h:80:17: warning: 'QV4::Managed* QV4::Value::asManaged() const'
redeclared without dllimport attribute after being referenced with dll
linkage
                 ^
qv4value_p.h:180:14: warning:
'static QV4::Value QV4::Value::fromManaged(QV4::Managed*)' redeclared
without dllimport attribute after being referenced with dll linkage
              ^
qv4value_p.h:285:16: warning:
'QV4::String* QV4::Value::asString() const' redeclared without dllimport
attribute after being referenced with dll linkage

Change-Id: I548a2f8049b8eca06ab1061f56416a332820dc01
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>