Include <cerrno>

When compiling on Linux host to armv7a target following to the tutorial, got error of "errno undeclared". Include this header fixed this problem.

Author: ppwwyyxx

test_helpers/Utils.cpp

@@ -26,7 +26,7 @@
 #include <cctype>
 #include <iomanip>
 #include <string>
-
+#include <cerrno>
 namespace
 {
 /* Advance the iterator to the first character which is not a comment
@@ -161,4 +161,4 @@ std::tuple<unsigned int, unsigned int, int> test_helpers::parse_ppm_header(std::
     fs.ignore(1);
 
     return std::make_tuple(width, height, max_val);
-}
+}