[openmp] [test] Fix building the affinity/format/fields_values.c testcase on Windows

[mstorsjo]

Summary

Add a missing <process.h> include for _getpid. Don't typedef the
pid_t type on mingw, as mingw headers already provide a typedef for
it.

openmp/runtime/test/affinity/format/fields_values.c

#define check(condition)                                                       \
   if (!(condition)) {                                                          \
     fprintf(stderr, "error: %s: %d: " STR(condition) "\n", __FILE__,           \
             __LINE__);                                                         \
     exit(1);                                                                   \
   }
 
 #if defined(_WIN32)
 #include <windows.h>
+#include <process.h>
 #define getpid _getpid
+#ifndef __MINGW32__
 typedef int pid_t;
+#endif
 #define gettid GetCurrentThreadId
 #define my_gethostname(buf, sz) GetComputerNameA(buf, &(sz))
 #else
 #include <unistd.h>
 #include <sys/types.h>
 #define my_gethostname(buf, sz) gethostname(buf, sz)
 #endif