Don't hardcode min Str in losestr

Min Str is typically 3 no matter the hero's race, but could be higher
(at least in theory?).  Using ATTRMIN makes losestr respect the same
minimum as other kinds of attribute loss (I'm operating under the
assumption that this wasn't an intentional move to fix the minimum at 3
regardless of other factors).

src/attrib.c

@@ -222,7 +222,7 @@ losestr(int num, const char *knam, schar k_format)
         k_format = KILLED_BY;
     }
 
-    while (ustr < 3) {
+    while (ustr < ATTRMIN(A_STR)) {
         ++ustr;
         --num;
         losehp(6, knam, k_format);