seed-os.c: include util.h for openpty() on macOS

On macOS, openpty() is defined in util.h, which needs to be included to avoid an implicit function declaration warning (which is now considered an error in Xcode 12):

libtool: compile:  /usr/bin/clang -arch x86_64 -DHAVE_CONFIG_H -I. -I../.. -I/opt/local/include -DBUILDING_GTK__ -isysroot/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk -I../../libseed/ -D_REENTRANT -I/opt/local/include/gobject-introspection-1.0 -I/opt/local/include/glib-2.0 -I/opt/local/lib/glib-2.0/include -DG_DISABLE_ASSERT -DG_DISABLE_CHECKS -DG_DISABLE_CAST_CHECKS -pipe -Os -isysroot/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk -arch x86_64 -Wall -MT libseed_os_la-seed-os.lo -MD -MP -MF .deps/libseed_os_la-seed-os.Tpo -c seed-os.c  -fno-common -DPIC -o .libs/libseed_os_la-seed-os.o
seed-os.c:813:5: error: implicit declaration of function 'openpty' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
    openpty(&master, &slave, NULL, NULL, NULL);
    ^
seed-os.c:813:5: note: did you mean 'openat'?
/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/sys/fcntl.h:532:9: note: 'openat' declared here
int     openat(int, const char *, int, ...) __DARWIN_NOCANCEL(openat) __OSX_AVAILABLE_STARTING(__MAC_10_10, __IPHONE_8_0);
        ^
1 error generated.