instrument functions: Satisfy bfd inclusion guard

In 2012 [0] binutils added an inclusion guard to bfd.h, requiring to
include the automake config.h beforehand. This guard is triggered when
enabling instrumentation and development mode:

```
% git clone https://github.com/the-tcpdump-group/tcpdump.git
% cd tcpdump
% touch .devel
% ./autogen.sh
% ./configure --enable-instrument-functions --enable-smb --quiet
./mkdep -c gcc -m -M -s . -DHAVE_CONFIG_H -I. fptype.c tcpdump.c print-smb.c smbutil.c instrument-functions.c <libnetdissect src list>
In file included from ./instrument-functions.c:18:
/usr/include/bfd.h:35:2: error: #error config.h must be included before this header
   35 | #error config.h must be included before this header
      |  ^~~~~
```

Note than this does *not* happen on Debian-based systems, because they
remove [1] that guard since 2012 [2]. That's also why the Ubuntu-based
Linux CI is not affected.

[0] https://sourceware.org/git/?p=binutils-gdb.git;a=blobdiff;f=bfd/bfd-in.h;h=ae8149a2;hp=d88bcb6c;hb=df7b86aa;hpb=134fa82e
[1] https://salsa.debian.org/toolchain-team/binutils/-/blob/38415eb8/debian/rules#L1045-1048
[2] https://code.launchpad.net/~doko/binutils/pkg-2.23-debian

Author: corubba

instrument-functions.c

@@ -11,6 +11,8 @@
  * FOR A PARTICULAR PURPOSE.
  */
 
+#include <config.h>
+
 #include <stdio.h>
 #include <string.h>
 #include <stdlib.h>