Crystal: always install pkg-config

[zegl]

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• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
• Does your build pass brew audit --strict <formula> (after doing brew install <formula>)?

---

pkg-config seems to always be required to compile Crystal programs,
see crystal-lang/crystal#6875

[igas]

Error Message
failed: brew audit crystal --online
Stacktrace
        Error: 6 problems in 1 formula detected
crystal:
  * C: 31: col 3: dependency "bdw-gc" (line 31) should be put before dependency "pkg-config" (line 30)
  * C: 32: col 3: dependency "gmp" (line 32) should be put before dependency "pkg-config" (line 30)
  * C: 33: col 3: dependency "libevent" (line 33) should be put before dependency "pkg-config" (line 30)
  * C: 34: col 3: dependency "libyaml" (line 34) should be put before dependency "pkg-config" (line 30)
  * C: 35: col 3: dependency "llvm@6" (line 35) should be put before dependency "pkg-config" (line 30)
  * C: 36: col 3: dependency "pcre" (line 36) should be put before dependency "pkg-config" (line 30)