Allow bootstrapping libopenbsd on Linux

The getdtablecount.c file won't compile on Linux but it seems like none of
the bootstrap tools actually need it.

Reviewed By:	emaste, brooks
Differential Revision: https://reviews.freebsd.org/D14244

lib/libopenbsd/Makefile

@@ -2,10 +2,15 @@
 
 PACKAGE=lib${LIB}
 LIB=	openbsd
-SRCS=	getdtablecount.c \
-	imsg-buffer.c \
+SRCS=	imsg-buffer.c \
 	imsg.c \
 	ohash.c
+.if !defined(BOOTSTRAPPING)
+# Skip getdtablecount.c when bootstrapping since it doesn't compile for Linux
+# and is not used by any of the bootstrap tools
+SRCS+=	getdtablecount.c
+.endif
+
 INTERNALLIB=
 
 CFLAGS+= -I${.CURDIR}