fs.c: Fix implicit declaration of function 'mkstemp'

system/core/libcutils/fs.c: In function 'fs_write_atomic_int':
system/core/libcutils/fs.c:109:5: warning: implicit declaration of function 'mkstemp' [-Wimplicit-function-declaration]

Change-Id: I337ecb818f85e20b058f782df45987acf2e9d6c1