nvs_flash: Fix compilation of host testcases when g++-8.x is used.

When std::bind is used, it requires inclusion of <functional> header.
This was not mandatory with earlier versions of g++ (4.x), may be because they
had experimental support for c++11.

components/nvs_flash/test_nvs_host/test_spi_flash_emulation.cpp

@@ -14,6 +14,7 @@
 #include "catch.hpp"
 #include "esp_spi_flash.h"
 #include "spi_flash_emulation.h"
+#include <functional>
 
 using namespace std;