[Backtracing] Only include libproc.h if it's present.

rdar://104336548

Author: al45tair

stdlib/public/SwiftShims/swift/shims/_SwiftBacktracing.h

@@ -29,10 +29,19 @@
 
 #include <CoreFoundation/CFUUID.h>
 #include <CoreFoundation/CFString.h>
-#endif
 
-#if TARGET_OS_OSX
+#if __has_include(<libproc.h>)
 #include <libproc.h>
+#else
+#ifdef __cplusplus
+extern "C" {
+#endif
+extern int proc_name(int pid, void * buffer, uint32_t buffersize);
+#ifdef __cplusplus
+}
+#endif
+#endif
+
 #endif
 
 #ifdef __cplusplus