net-snmp: support compilation with GCC 11 permissively

net-snmp: support compilation with GCC 11 permissively

Use #elif defined instead of #elifdef as seen elsewhere throughout the patch
file. This avoids the following errors when compiling with GCC 11:

```
mibgroup/ucd-snmp/proc.c:45:2: error: invalid preprocessing directive #elifdef; did you mean #ifdef?
   45 | #elifdef HAVE_PCRE_H
      |  ^~~~~~~
      |  ifdef

mibgroup/ucd-snmp/proc.c:243:2: error: invalid preprocessing directive #elifdef; did you mean #ifdef?
  243 | #elifdef HAVE_PCRE_H
      |  ^~~~~~~
      |  ifdef
```

Signed-off-by: Wei-Ting Yang <williamatcg@gmail.com>