another attempt at fixing the examples in the error codes .md

Author: folkertdev

compiler/rustc_error_codes/src/error_codes/E0798.md

@@ -14,6 +14,7 @@ Erroneous code example:
 ```compile_fail,E0798
 #![feature(abi_c_cmse_nonsecure_call)]
 
+#[no_mangle]
 pub fn test(
     f: extern "C-cmse-nonsecure-call" fn(u32, u32, u32, u32, u32) -> u32,
 ) -> u32 {
@@ -28,6 +29,7 @@ room left for the final `f32` argument
 ```compile_fail,E0798
 #![feature(abi_c_cmse_nonsecure_call)]
 
+#[no_mangle]
 pub fn test(
     f: extern "C-cmse-nonsecure-call" fn(u32, u64, f32) -> u32,
 ) -> u32 {