Build failed on OpenBSD 6.5

packetdrill 296fb3f046d5528b90d8b3a73e99075b1113a355

`uname -sr`: OpenBSD 6.5

```
$ ./configure
$ make

cc -g -Wall -Werror -c packetdrill.c
In file included from packetdrill.c:25:
In file included from ./types.h:43:
./platforms.h:49:9: error: unknown type name 'u8'
typedef u8 __u8;
^
./platforms.h:50:9: error: unknown type name 'u16'
typedef u16 __u16;
^
./platforms.h:51:9: error: unknown type name 'u32'
typedef u32 __u32;
^
./platforms.h:52:9: error: unknown type name 'u64'
typedef u64 __u64;
^
./platforms.h:55:9: error: unknown type name 'u16'
typedef u16 __le16;
^
./platforms.h:56:9: error: unknown type name 'u16'
typedef u16 __be16;
^
./platforms.h:57:9: error: unknown type name 'u32'
typedef u32 __le32;
^
./platforms.h:58:9: error: unknown type name 'u32'
typedef u32 __be32;
^
./platforms.h:59:9: error: unknown type name 'u64'
typedef u64 __le64;
^
./platforms.h:60:9: error: unknown type name 'u64'
typedef u64 __be64;
^
./platforms.h:62:9: error: unknown type name 'u16'
typedef u16 __sum16;
^
./platforms.h:63:9: error: unknown type name 'u32'
typedef u32 __wsum;
^
In file included from packetdrill.c:25:
In file included from ./types.h:43:
In file included from ./platforms.h:97:
./fmemopen.h:33:14: error: conflicting types for 'fmemopen'
extern FILE *fmemopen(char *buf, size_t size, const char *mode);
^
/usr/include/stdio.h:330:7: note: previous declaration is here
FILE *fmemopen(void *, size_t, const char *);
^
In file included from packetdrill.c:35:
In file included from ./run.h:71:
In file included from ./so_testing.h:28:
./packetdrill.h:37:10: fatal error: 'sys/epoll.h' file not found
#include
^~~~~~~~~~~~~
14 errors generated.
*** Error 1 in /home/sergeyb/source/packetdrill/gtests/net/packetdrill (:87 'packetdrill.o')
```