strtod may return HUGE_VAL on overflow, which is defined in math.h.

Therefore added math.h to SYNPOSIS.
OK millert@

src/lib/libc/stdlib/strtod.3

@@ -29,9 +29,9 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.\"	$OpenBSD: strtod.3,v 1.9 2007/05/31 19:19:31 jmc Exp $
+.\"	$OpenBSD: strtod.3,v 1.10 2007/11/11 12:23:36 tobias Exp $
 .\"
-.Dd $Mdocdate: May 31 2007 $
+.Dd $Mdocdate: November 11 2007 $
 .Dt STRTOD 3
 .Os
 .Sh NAME
@@ -40,6 +40,7 @@
 .Tn ASCII
 string to double
 .Sh SYNOPSIS
+.Fd #include <math.h>
 .Fd #include <stdlib.h>
 .Ft double
 .Fn strtod "const char *nptr" "char **endptr"