linux: Fix warning when compiling with meson

src/up-input.c:294:3: warning: implicit declaration of function ‘close’; did you mean ‘pclose’? [-Wimplicit-function-declaration]
  294 |   close (input->eventfp);
      |   ^~~~~
      |   pclose

src/linux/up-input.c

@@ -23,6 +23,7 @@
 #endif
 
 #include <errno.h>
+#include <unistd.h>
 #include <fcntl.h>
 #include <linux/input.h>