build fails with musl libc

[tomalok]

commits 16612d7 and af9da0a have broken builds on musl libc systems.

# github.com/opencontainers/runc/libcontainer/nsenter
/usr/lib/gcc/x86_64-alpine-linux-musl/8.3.0/../../../../x86_64-alpine-linux-musl/bin/ld: $WORK/b123/_x003.o: in function `try_bindfd':
libcontainer/nsenter/cloned_binary.c:354: undefined reference to `secure_getenv'
/usr/lib/gcc/x86_64-alpine-linux-musl/8.3.0/../../../../x86_64-alpine-linux-musl/bin/ld: $WORK/b123/_x003.o: in function `make_execfd':
libcontainer/nsenter/cloned_binary.c:252: undefined reference to `secure_getenv'
collect2: error: ld returned 1 exit status
# github.com/opencontainers/runc/libcontainer/nsenter
cloned_binary.c: In function 'make_execfd':
cloned_binary.c:252:17: warning: implicit declaration of function 'secure_getenv' [-Wimplicit-function-declaration]
  char *prefix = secure_getenv("_LIBCONTAINER_STATEDIR");
                 ^~~~~~~~~~~~~
cloned_binary.c:252:17: warning: initialization of 'char *' from 'int' makes pointer from integer without a cast [-Wint-conversion]
cloned_binary.c: In function 'try_bindfd':
cloned_binary.c:354:17: warning: initialization of 'char *' from 'int' makes pointer from integer without a cast [-Wint-conversion]
  char *prefix = secure_getenv("_LIBCONTAINER_STATEDIR");
                 ^~~~~~~~~~~~~
make: *** [Makefile:33: runc] Error 2

[cyphar]

This was fixed by #2015.