build: Fix strndup detection on MinGW

Joshua Watt requested to merge jpewhacker/wayland:mingw-fixes into main

GCC and meson conspire together to incorrectly detect that strndup() exists on MinGW as __builtin_strndup, when no such function exists. As a work around, meson will skip looking for __builtin functions if an '#include' is in the prefix, so add '#include <string.h>' when looking for strndup().


Signed-off-by: Joshua Watt

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