[llvm-exegesis] Use correct pid_t definition in SubprocessMemory.h

Due to failures in MinGW builds I adjusted the preprocessor directives
to be more specific about when to include but kept the old definition in
files that were committed more recently on accident. This patch changes
those definitions to match the fixed ones.

Also fix a typo leftover from the original change in PerfHelper.h.

llvm/tools/llvm-exegesis/lib/PerfHelper.h

@@ -27,7 +27,7 @@
 typedef int pid_t;
 #else
 #include <sys/types.h>
-#endif // HAVE_LIBPFM
+#endif // _MSC_VER
 
 struct perf_event_attr;
 

llvm/tools/llvm-exegesis/lib/SubprocessMemory.h

@@ -20,9 +20,12 @@
 #include <unordered_map>
 #include <vector>
 
-#ifndef __linux__
+#ifdef _MSC_VER
 typedef int pid_t;
-#endif // __linux__
+#else
+#include <sys/types.h>
+#endif // _MSC_VER
+
 
 namespace llvm {
 namespace exegesis {