Conditionally compile fn strlen

`strlen` is only used under certain feature flags, use `cfg` to
conditionally build it in.

Clears clippy warning.

Author: tcharding

secp256k1-sys/src/lib.rs

@@ -630,7 +630,8 @@ pub unsafe extern "C" fn rustsecp256k1_v0_3_1_default_error_callback_fn(message:
     panic!("[libsecp256k1] internal consistency check failed {}", msg);
 }
 
-
+#[no_mangle]
+#[cfg(not(feature = "external-symbols"))]
 unsafe fn strlen(mut str_ptr: *const c_char) -> usize {
     let mut ctr = 0;
     while *str_ptr != '\0' as c_char {
@@ -1161,11 +1162,13 @@ pub use self::fuzz_dummy::*;
 
 #[cfg(test)]
 mod tests {
-    use std::ffi::CString;
-    use super::strlen;
-
+    #[no_mangle]
+    #[cfg(not(feature = "external-symbols"))]
     #[test]
     fn test_strlen() {
+        use std::ffi::CString;
+        use super::strlen;
+
         let orig = "test strlen \t \n";
         let test = CString::new(orig).unwrap();