Declare new hash function in namespace std in order to fix build.

Fixes:

.../src/analyze/gui/parser.cpp:37:8: error: explicit specialization of ‘template<class _Tp> struct std::hash’ outside its namespace must use a nested-name-specifier [-fpermissive]

37 | struct hash<QString> {
   |        ^~~~~~~~~~~~~

seen with GCC 9.2.0 on Arch Linux.

Reviewers: dvratil, mwolff
Reviewed By: mwolff
Differential Revision: https://phabricator.kde.org/D24532