depends: define `__BSD_VISIBLE` for FreeBSD bdb build

Required for additional definitions (IPC_R & friends), to be available,
when compiling under C11.

See: https://github.com/MarcoFalke/btc_nightly/pull/4.

depends/packages/bdb.mk

@@ -15,7 +15,7 @@ $(package)_config_opts_netbsd=--with-pic
 $(package)_config_opts_openbsd=--with-pic
 $(package)_config_opts_android=--with-pic
 $(package)_cflags+=-Wno-error=implicit-function-declaration -Wno-error=format-security
-$(package)_cppflags_freebsd=-D_XOPEN_SOURCE=600
+$(package)_cppflags_freebsd=-D_XOPEN_SOURCE=600 -D__BSD_VISIBLE=1
 $(package)_cppflags_netbsd=-D_XOPEN_SOURCE=600
 $(package)_cppflags_openbsd=-D_XOPEN_SOURCE=600
 $(package)_cppflags_mingw32=-DUNICODE -D_UNICODE