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>
+#ifdef HAVE_PCRE2_H
+#define PCRE2_CODE_UNIT_WIDTH 8
+#include <pcre2.h>
-+#elifdef HAVE_PCRE_H
++#elif defined(HAVE_PCRE_H)
#include <pcre.h>
#endif
+ if ((*procp)->regexp.regex_ptr == NULL) {
+ config_perror(pcre2_error_msg);
+ }
-+#elifdef HAVE_PCRE_H
++#elif defined(HAVE_PCRE_H)
const char *pcre_error;
int pcre_error_offset;