host/lib/include/subprocess.h: Add `extern` to globals

This prevents linker errors regarding multiple definitions.

Change-Id: I62d6132c20ca2b9620a9f3b1539a1a0109c5aa26
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/2318026
Reviewed-by: Jack Rosenthal <jrosenth@chromium.org>
Tested-by: Joel Kitching <kitching@chromium.org>
Commit-Queue: Jack Rosenthal <jrosenth@chromium.org>
diff --git a/host/lib/include/subprocess.h b/host/lib/include/subprocess.h
index eaf7550..b24b987 100644
--- a/host/lib/include/subprocess.h
+++ b/host/lib/include/subprocess.h
@@ -85,22 +85,22 @@
 /**
  * A convenience subprocess target which uses TARGET_NULL.
  */
-struct subprocess_target subprocess_null;
+extern struct subprocess_target subprocess_null;
 
 /**
  * A convenience subprocess target which uses TARGET_FD to STDIN_FILENO.
  */
-struct subprocess_target subprocess_stdin;
+extern struct subprocess_target subprocess_stdin;
 
 /**
  * A convenience subprocess target which uses TARGET_FD to STDOUT_FILENO.
  */
-struct subprocess_target subprocess_stdout;
+extern struct subprocess_target subprocess_stdout;
 
 /**
  * A convenience subprocess target which uses TARGET_FD to STDERR_FILENO.
  */
-struct subprocess_target subprocess_stderr;
+extern struct subprocess_target subprocess_stderr;
 
 /**
  * Call a process and run until completion.