curses A_ITALIC build fix

My /usr/include/curses.h has various A_attribute macros but A_ITALIC
isn't one of them.  Compiling cursmisc.c failed because one of the
uses of that wasn't guarded by #ifdef A_ITALIC.  Instead of adding the
ommitted #if, substitute A_UNDERLINE for A_ITALIC when that's missing.

The select attribute menu when adding a menu color or a status hilite
now shows an entry for italic that's underlined (as expected) but the
underline entry itself does not display any sort of attribute.  I
didn't pursue that.

win/curses/cursmisc.c

@@ -12,6 +12,10 @@
 
 #include <ctype.h>
 
+#ifndef A_ITALIC
+#define A_ITALIC A_UNDERLINE
+#endif
+
 /* Misc. curses interface functions */
 
 /* Private declarations */