[check-system] Make zircon/system/ulib/mini-process check-system clean

mini-process and subprocess are circularly dependent because of the
toolchain fussing, so just add mini-process.h to subprocess as well.

    ERROR at //zircon/system/ulib/mini-process/subprocess.c:9:11: Include not allowed.
    #include <mini-process/mini-process.h>
              ^--------------------------
    It is not in any dependency of
      //zircon/system/ulib/mini-process:subprocess
    The include file is in the target(s):
      //zircon/system/ulib/mini-process:mini-process
    which should somehow be reachable.

Bug: 55166
Change-Id: I24d07186e2878250c75ed2e6e901b8ec05141d2c
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/404904
Commit-Queue: Scott Graham <scottmg@google.com>
Reviewed-by: John Grossman <johngro@google.com>
diff --git a/zircon/system/ulib/mini-process/BUILD.gn b/zircon/system/ulib/mini-process/BUILD.gn
index c4cf148..aa2184e 100644
--- a/zircon/system/ulib/mini-process/BUILD.gn
+++ b/zircon/system/ulib/mini-process/BUILD.gn
@@ -41,7 +41,10 @@
 
     # TODO(44278): Revert to true when converting this file to proper GN build idioms.
     testonly = false
-    sources = [ "subprocess.c" ]
+    sources = [
+      "include/mini-process/mini-process.h",
+      "subprocess.c",
+    ]
     deps = [ "//zircon/system/ulib/backtrace-request" ]
 
     configs += [