strcasecompare: include curl.h in strcase.c

This should fix the "warning: 'curl_strequal' redeclared without
dllimport attribute: previous dllimport ignored" message and subsequent
link error on Windows because of the missing CURL_EXTERN on the
prototype.

lib/strcase.c

@@ -21,6 +21,9 @@
  ***************************************************************************/
 
 #include "curl_setup.h"
+
+#include <curl/curl.h>
+
 #include "strcase.h"
 
 /* Portable, consistent toupper (remember EBCDIC). Do not use toupper() because