Rename ProcResult stdout and stderr fields

stdout and stderr are macros in musl, which causes build failures
when they are used as field names.  Rename the fields to stdout_str
and stderr_str.

Test: builds
Change-Id: I04770d2d6e82ca8393308ab12a304838a80f0ae5
diff --git a/libs/androidfw/PosixUtils.cpp b/libs/androidfw/PosixUtils.cpp
index 4ec525a..0269128 100644
--- a/libs/androidfw/PosixUtils.cpp
+++ b/libs/androidfw/PosixUtils.cpp
@@ -114,10 +114,10 @@
       std::unique_ptr<ProcResult> result(new ProcResult());
       result->status = status;
       const auto out = ReadFile(stdout[0]);
-      result->stdout = out ? *out : "";
+      result->stdout_str = out ? *out : "";
       close(stdout[0]);
       const auto err = ReadFile(stderr[0]);
-      result->stderr = err ? *err : "";
+      result->stderr_str = err ? *err : "";
       close(stderr[0]);
       return result;
   }