use relative path in # include (#1194)

Reverts commit that changes #include "assert" -> #include "gsl/assert".
This change is necessary in order to comply with CppCoreGuideline's
SF.12. Now we do #include "./assert".

include/gsl/pointers

@@ -17,7 +17,7 @@
 #ifndef GSL_POINTERS_H
 #define GSL_POINTERS_H
 
-#include "gsl/assert" // for Ensures, Expects
+#include "./assert" // for Ensures, Expects
 
 #include <cstddef>      // for ptrdiff_t, nullptr_t, size_t
 #include <functional>   // for less, greater