build: Some compiler warnings with musl

[rusty-snake]

Some compiler warnings. Not critical but they will fail the build with --enable-fatal-warnings.

Firejail: 65c5480
gcc: gcc (Alpine 11.2.1_git20220219) 11.2.1 20220219

main.c:193: warning: "LINE_MAX" redefined
  193 | #define LINE_MAX 200
      | 
In file included from /usr/include/fortify/stdlib.h:29,
                 from ../include/common.h:27,
                 from fnettrace.h:23,
                 from main.c:20:
/usr/include/limits.h:84: note: this is the location of the previous definition
   84 | #define LINE_MAX 4096
      | 

libtracelog.c:43:23: warning: 'orig_fopen64' defined but not used [-Wunused-variable]
   43 | static orig_fopen64_t orig_fopen64 = NULL;
      |                       ^~~~~~~~~~~~

libtrace.c:48:23: warning: 'orig_fopen64' defined but not used [-Wunused-variable]
   48 | static orig_fopen64_t orig_fopen64 = NULL;
      |  

[reinerh]

Interesting that it doesn't happen in the CI, where fatal warnings are enabled.

[reinerh]

The difference to our CI is that Alpine builds with musl.

@rusty-snake Can you please try building on Alpine with #5431?