Only include dispatch.h when needed

[evanmiller]

This change allows "small" vim to be built on pre-dispatch Mac systems (10.5 and earlier).

[codecov]

Codecov Report

Merging #10954 (8cffc93) into master (b82a2ab) will decrease coverage by 0.87%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #10954      +/-   ##
==========================================
- Coverage   82.69%   81.81%   -0.88%     
==========================================
  Files         148      158      +10     
  Lines      173784   186480   +12696     
  Branches    39271    42157    +2886     
==========================================
+ Hits       143706   152566    +8860     
- Misses      17479    21417    +3938     
+ Partials    12599    12497     -102     

Flag	Coverage Δ
huge-clang-none	82.68% <ø> (-0.01%)	⬇️
linux	82.68% <ø> (-0.01%)	⬇️
mingw-x64-HUGE	0.00% <ø> (?)
mingw-x64-HUGE-gui	78.21% <ø> (?)
windows	76.99% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/sound.c	64.36% <0.00%> (-21.07%)	⬇️
src/if_python3.c	72.54% <0.00%> (-7.90%)	⬇️
src/if_cscope.c	68.65% <0.00%> (-7.05%)	⬇️
src/locale.c	75.43% <0.00%> (-6.38%)	⬇️
src/terminal.c	77.39% <0.00%> (-4.90%)	⬇️
src/libvterm/src/keyboard.c	42.39% <0.00%> (-3.96%)	⬇️
src/libvterm/src/unicode.c	84.00% <0.00%> (-3.50%)	⬇️
src/libvterm/src/pen.c	47.60% <0.00%> (-3.42%)	⬇️
src/mbyte.c	72.63% <0.00%> (-3.15%)	⬇️
src/libvterm/src/mouse.c	41.93% <0.00%> (-2.90%)	⬇️
... and 127 more

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