Fix warn -Wswitch + message for 'GIMP_ZOOM_PINCH’
Fix:
[2119/2321] Compiling C object app/widgets/libappwidgets.a.p/gimpgradienteditor.c.o
../app/widgets/gimpgradienteditor.c: In function ‘gimp_gradient_editor_zoom’:
../app/widgets/gimpgradienteditor.c:685:3: warning: enumeration value ‘GIMP_ZOOM_PINCH’ not handled in switch [-Wswitch]
685 | switch (zoom_type)
| ^~~~~~
No problem to squash/roll'up with other MR. Indirectly to: #7251 (closed)