try to fix exif feature build on Linux

src/_exif.rs

@@ -2,4 +2,5 @@
 #![allow(non_snake_case)]
 #![allow(dead_code)]
 #![allow(deref_nullptr)]
+#![allow(non_upper_case_globals)]
 include!(concat!(env!("OUT_DIR"), "/exif.rs"));

src/exif.rs

@@ -477,7 +477,7 @@ impl ExifValue {
         if n >= c {
             return None;
         }
-        let r = unsafe { _exif::exif_value_to_int64(self.value, n as i32) };
+        let r = unsafe { _exif::exif_value_to_int64(self.value, n as c_long) };
         if !self.ok() {
             return None;
         }