Add extern "C" to header process_shims.h

Previous, we would encounter missing symbol errors like:

          "_subprocess_vm_size()", referenced from:
              closure #1 () -> Swift.Int in variable initialization expression of Subprocess.(_pageSize in _EDE99368FF43462E0C75AF3C9D4D8F2D) : Swift.Int in AsyncBufferSequence.swift.o
           NOTE: found '__subprocess_vm_size' in process_shims.c.o, declaration possibly missing 'extern "C"'

We can reproduce this by running:

        xcrun swift build -Xswiftc -cxx-interoperability-mode=default --build-tests

Adding `extern "C"` fixes these issues.

rdar://161624906

Author: j-hui

Sources/_SubprocessCShims/include/process_shims.h

@@ -35,6 +35,10 @@
 #include <sys/wait.h>
 #endif // TARGET_OS_LINUX || TARGET_OS_FREEBSD
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #if __has_include(<mach/vm_page_size.h>)
 vm_size_t _subprocess_vm_size(void);
 #endif
@@ -102,12 +106,20 @@ int _pidfd_open(pid_t pid);
 
 #endif
 
+#ifdef __cplusplus
+} // extern "C"
+#endif
+
 #endif // !TARGET_OS_WINDOWS
 
 #if TARGET_OS_WINDOWS
 
 #include <Windows.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #ifndef _WINDEF_
 typedef unsigned long DWORD;
 typedef int BOOL;
@@ -122,6 +134,10 @@ unsigned int _subprocess_windows_get_errno(void);
 /// complex macro and cannot be imported directly into Swift.
 DWORD_PTR _subprocess_PROC_THREAD_ATTRIBUTE_HANDLE_LIST(void);
 
+#ifdef __cplusplus
+} // extern "C"
+#endif
+
 #endif
 
 #endif /* process_shims_h */