Fix the following compile error

fusermount.c: In function 'clone_newns':
fusermount.c:315:2: warning: implicit declaration of function 'clone' [-Wimplicit-function-declaration]
fusermount.c:315:44: error: 'CLONE_NEWNS' undeclared (first use in this function)
fusermount.c:315:44: note: each undeclared identifier is reported only once for each function it appears in
fusermount.c:317:1: warning: control reaches end of non-void function [-Wreturn-type]
1 file changed