Suppress invalid stringop-truncation warnings

GCC tries to detect possible losses of the null terminator; see e.g.
https://developers.redhat.com/blog/2018/05/24/detecting-string-truncation-with-gcc-8/

In this case we already check if the null terminator is missing, and exit the program, but GCC does not detect this workaround.

Just suppress these warnings using #pragma

Before
------
Build error on Fedora 31
```
In file included from /usr/include/string.h:495,
                 from /usr/include/glib-2.0/glib/gtestutils.h:30,
                 from /usr/include/glib-2.0/glib.h:85,
                 from helper.h:18,
                 from helper.c:15:
In function 'strncpy',
    inlined from 'main' at helper.c:404:9:
/usr/include/bits/string_fortified.h:106:10: error: '__builtin_strncpy' specified bound 100 equals destination size [-Werror=stringop-truncation]
  106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In function 'strncpy',
    inlined from 'main' at helper.c:411:17:
/usr/include/bits/string_fortified.h:106:10: error: '__builtin_strncpy' specified bound 100 equals destination size [-Werror=stringop-truncation]
  106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
m
```

After
-----
Builds fine

src/helper.c

@@ -383,7 +383,11 @@ int main(int argc, char *argv[]) {
         }
 
         syslog(LOG_DEBUG, "Reading argument 1");
-        strncpy(state->crypt_mapper_name, argv[1], STRING_BUFFER_SIZE);
+        // suppress stringop-truncatation warning
+        // we already check for missing \0
+        #pragma GCC diagnostic push
+        #pragma GCC diagnostic ignored "-Wstringop-truncation"
+        (strncpy(state->crypt_mapper_name, argv[1], STRING_BUFFER_SIZE));
         if (state->crypt_mapper_name[STRING_BUFFER_SIZE - 1] != 0) {
                 syslog(LOG_ERR, "Crypt device name overran buffer size of %d bytes.", STRING_BUFFER_SIZE);
                 exit(EX_SOFTWARE);
@@ -415,6 +419,7 @@ int main(int argc, char *argv[]) {
                 }
                 syslog(LOG_INFO, "Setting cache path to %s", state->cache_path);
         }
+        # pragma GCC diagnostic pop
 
         syslog(LOG_DEBUG, "Reading password from login module.");
         if (fgets(state->password, PASSWORD_BUFFER_SIZE, stdin) == NULL) {