sal android: fix -Werror,-Wimplicit-function-declaration

Change-Id: If19dbb654d473e8785dc69f96775c78cc95a7bd6

sal/android/libreofficekit-jni.c

@@ -17,6 +17,7 @@
 #include <sys/time.h>
 #include <sys/types.h>
 #include <time.h>
+#include <unistd.h>
 
 #include <jni.h>