Fix fcntl for FreeBSD

Need to use the right cfg option for the conditional compilation.
target_os is the right option to use when targeting FreeBSD. target_env
was used before which seems to be a typo.

Author: Alexander Thaller

src/fcntl.rs

@@ -20,7 +20,7 @@ use crate::sys::uio::IoVec; // For vmsplice
     target_os = "fuchsia",
     any(target_os = "wasi", target_env = "wasi"),
     target_env = "uclibc",
-    target_env = "freebsd"
+    target_os = "freebsd"
 ))]
 pub use self::posix_fadvise::*;
 
@@ -587,7 +587,7 @@ pub fn fallocate(
     target_os = "fuchsia",
     any(target_os = "wasi", target_env = "wasi"),
     target_env = "uclibc",
-    target_env = "freebsd"
+    target_os = "freebsd"
 ))]
 mod posix_fadvise {
     use crate::errno::Errno;

test/test_fcntl.rs

@@ -332,7 +332,7 @@ mod linux_android {
           target_os = "fuchsia",
           any(target_os = "wasi", target_env = "wasi"),
           target_env = "uclibc",
-          target_env = "freebsd"))]
+          target_os = "freebsd"))]
 mod test_posix_fadvise {
 
     use tempfile::NamedTempFile;