Fix crypt.cpp sanitizer test on FreeBSD

[arichardson]

Summary

FreeBSD doesn't provide a crypt.h header but instead defines the functions
in unistd.h. Use __has_include() to handle that case.

compiler-rt/test/sanitizer_common/TestCases/Posix/crypt.cpp

 // RUN: %clangxx -O0 -g %s -o %t -lcrypt && %run %t
 
 // crypt() is missing from Android and -lcrypt from darwin.
 // UNSUPPORTED: android, darwin
 
 #include <assert.h>
 #include <unistd.h>
 #include <cstring>
+#if __has_include(<crypt.h>)
 #include <crypt.h>
+#endif
 
 int
 main (int argc, char** argv)
 {
   {
     char *p = crypt("abcdef", "xz");
     volatile size_t z = strlen(p);
   }