tests/strerror_override.c: fix compilation error

I got this on Mac OS X at least.
Not sure if it shows up in other envs

error:
```
strerror_override.c:53:13: error: incompatible redeclaration of library function 'strerror' [-Werror,-Wincompatible-library-redeclaration]
const char *strerror(int errno_in)
```

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

tests/strerror_override.c

@@ -50,7 +50,7 @@ static struct {
 
 #define PREFIX "ERRNO="
 static char errno_buf[128] = PREFIX;
-const char *strerror(int errno_in)
+char *strerror(int errno_in)
 {
 	int start_idx;
 	char digbuf[20];