graphics/sekrit-twc-zimg: use bundled math library to unbreak some tests

[==========] 129 tests from 25 test suites ran. (29301 ms total)
[ PASSED ] 127 tests.
[ FAILED ] 2 tests, listed below:
[ FAILED ] ColorspaceConversionTest.test_transfer_only_b67
[ FAILED ] ColorspaceConversionTest.test_rec2100_ictcp

PR: 216537