Fix the build in clang

"fprintf(stderr, buf)" is a warning in some compilers, and we're
building with all warnings promoted to error.

Change-Id: Ie3c6ddcd74cf4dda40cf7b742df955dde1d1a5ff
diff --git a/commandline.c b/commandline.c
index f8e9843..96ea8c6 100644
--- a/commandline.c
+++ b/commandline.c
@@ -1994,7 +1994,7 @@
     }
     if (session_id < 0) {
         fprintf(stderr, "Failed to create session\n");
-        fprintf(stderr, buf);
+        fputs(buf, stderr);
         return -1;
     }
 
@@ -2034,7 +2034,7 @@
 
         if (strncmp("Success", buf, 7)) {
             fprintf(stderr, "Failed to write %s\n", file);
-            fprintf(stderr, buf);
+            fputs(buf, stderr);
             success = 0;
             goto finalize_session;
         }
@@ -2057,11 +2057,11 @@
     adb_close(fd);
 
     if (!strncmp("Success", buf, 7)) {
-        fprintf(stderr, buf);
+        fputs(buf, stderr);
         return 0;
     } else {
         fprintf(stderr, "Failed to finalize session\n");
-        fprintf(stderr, buf);
+        fputs(buf, stderr);
         return -1;
     }
 }