macOS SDK unit tests fail to link — mbgl::util::default_styles

[1ec5]

The macOS SDK unit test bundle fails to link with the following errors:

Undefined symbols for architecture x86_64:
  "mbgl::util::default_styles::satelliteStreets", referenced from:
      -[MGLStyleTests testVersionedStyleURLs] in MGLStyleTests.o
  "mbgl::util::default_styles::dark", referenced from:
      -[MGLStyleTests testVersionedStyleURLs] in MGLStyleTests.o
  "mbgl::util::default_styles::light", referenced from:
      -[MGLStyleTests testVersionedStyleURLs] in MGLStyleTests.o
  "mbgl::util::default_styles::streets", referenced from:
      -[MGLStyleTests testVersionedStyleURLs] in MGLStyleTests.o
  "mbgl::util::default_styles::outdoors", referenced from:
      -[MGLStyleTests testVersionedStyleURLs] in MGLStyleTests.o
  "mbgl::util::default_styles::satellite", referenced from:
      -[MGLStyleTests testVersionedStyleURLs] in MGLStyleTests.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Possibly fallout from #5359.

/cc @kkaefer