build: add support for OpenBSD to depends

depends/packages/libevent.mk

@@ -13,6 +13,7 @@ define $(package)_set_vars
   $(package)_config_opts_release=--disable-debug-mode
   $(package)_config_opts_linux=--with-pic
   $(package)_config_opts_freebsd=--with-pic
+  $(package)_config_opts_openbsd=--with-pic
   $(package)_config_opts_android=--with-pic
   $(package)_cppflags_mingw32=-D_WIN32_WINNT=0x0601
 endef