package/kodi: fix build with newer gcc versions.

Using this defconfig produces a build error with kodi:

BR2_arm=y
BR2_cortex_a8=y
BR2_ARM_INSTRUCTIONS_THUMB2=y
BR2_TOOLCHAIN_EXTERNAL=y
BR2_PACKAGE_KODI=y
BR2_PACKAGE_SUNXI_MALI=y
BR2_PACKAGE_PYTHON=y
BR2_PACKAGE_PYTHON_PY_ONLY=y

/home/buildroot/buildroot/output/build/kodi-17.3-Krypton/xbmc/filesystem/FTPParse.cpp:37:1:
 error: ‘string’ does not name a type
 string CFTPParse::getName()
 ^~~~~~
/home/buildroot/buildroot/output/build/kodi-17.3-Krypton/xbmc/filesystem/FTPParse.cpp:62:25:
 error: variable or field ‘setTime’ declared void
 void CFTPParse::setTime(string str)
                         ^~~~~~
/home/buildroot/buildroot/output/build/kodi-17.3-Krypton/xbmc/filesystem/FTPParse.cpp:62:25:
 error: ‘string’ was not declared in this scope

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>