Unbreak build on FreeBSD

libbsd looks unusable, so cannot be packaged.

$ meson setup _build
[...]
src/meson.build:47:2: ERROR: Dependency "libbsd" not found, tried pkgconfig

$ meson compile -C _build
[...]
../eek/eek-keyboard.c:24:10: fatal error: 'bsd/string.h' file not found
#include <bsd/string.h>
         ^~~~~~~~~~~~~~

#define _XOPEN_SOURCE 500 causes the following:

../eek/eek-keyboard.c:78:5: error: implicitly declaring library function 'strlcpy' with type 'unsigned long (char *, const char *, unsigned long)' [-Werror,-Wimplicit-function-declaration]
    strlcpy(ptr, xkb_keymap_str, keymap_len);
    ^
../eek/eek-keyboard.c:78:5: note: include the header <string.h> or explicitly provide a declaration for 'strlcpy'