Fix Kinetic speed condition

[xnuk]

Description

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

In #define MK_COMBINED, this compile error is shown:

quantum/mousekey.c: In function 'move_unit':
quantum/mousekey.c:135:35: error: 'mouse_timer' undeclared (first use in this function)
  135 |     } else if (mousekey_repeat && mouse_timer) {
      |                                   ^~~~~~~~~~~
quantum/mousekey.c:135:35: note: each undeclared identifier is reported only once for each function it appears in
quantum/mousekey.c: At top level:
quantum/mousekey.c:149:7: error: conflicting types for 'mk_wheel_interval'
  149 | float mk_wheel_interval = 1000.0f / MOUSEKEY_WHEEL_INITIAL_MOVEMENTS;
      |       ^~~~~~~~~~~~~~~~~
quantum/mousekey.c:68:9: note: previous definition of 'mk_wheel_interval' was here
   68 | uint8_t mk_wheel_interval    = MOUSEKEY_WHEEL_INTERVAL;
      |         ^~~~~~~~~~~~~~~~~
quantum/mousekey.c: In function 'wheel_unit':
quantum/mousekey.c:156:35: error: 'mouse_timer' undeclared (first use in this function)
  156 |     } else if (mousekey_repeat && mouse_timer) {
      |                                   ^~~~~~~~~~~

And the mouse_timer is defined at #ifdef MK_KINETIC_SPEED:

qmk_firmware/quantum/mousekey.c

Lines 39 to 41 in 788b185

	#ifdef MK_KINETIC_SPEED
	static uint16_t mouse_timer = 0;
	#endif

But, if I read correctly, mouse_timer is only used when #ifndef MK_KINETIC_SPEED.

This PR fixes this by flipping the condition.

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).