Fix a warning on FreeBSD

This should fix the following:

```C
snuffleupagus/src/sp_config.c:193:10: warning: implicit declaration of function 'getline' is invalid in C99 [-Wimplicit-function-declaration]
  while (getline(&lineptr, &n, fd) > 0) {
         ^
1 warning generated.
```

1 files changed, 3 insertions, 0 deletions

diff --git a/src/sp_config.c b/src/sp_config.c
index dbf3881..debf749 100644
--- a/src/sp_config.c
+++ b/src/sp_config.c
@@ -1,4 +1,7 @@
 #include <errno.h>
+#if defined(__FreeBSD__)
+#define _WITH_GETLINE
+#endif
 #include <stdio.h>
 #include <string.h>