Move mkdirs to adb_utils.

There were duplicate implementations in commandline.cpp and
file_sync_client.cpp.

Change-Id: Ib448f76c0d7ffdcd577336b1c610a881425bc2db
diff --git a/adb_utils.h b/adb_utils.h
index 84f7d0c..c816d00 100644
--- a/adb_utils.h
+++ b/adb_utils.h
@@ -22,6 +22,8 @@
 bool getcwd(std::string* cwd);
 bool directory_exists(const std::string& path);
 
+int mkdirs(const char *path);
+
 std::string escape_arg(const std::string& s);
 
 void dump_hex(const void* ptr, size_t byte_count);