build: Fix strndup detection on MinGW

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().

See: https://github.com/mesonbuild/meson/issues/3672

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
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