Fix build failure.

--- a/src/context.cpp
+++ b/src/context.cpp
@@ -33,7 +33,7 @@ CONFIG::~CONFIG() {
 bool CONFIG::find(const char *needle, string_set &haystack) {
 	string_set::iterator i = haystack.find(needle);
 	if (i != haystack.end()) return true;		   // found user@domain.tld key
-	char *x = strchr(needle, '@');
+	char *x = (char *) strchr(needle, '@');
 	if (x) {
 		x++;
 		i = haystack.find(x);
@@ -51,7 +51,7 @@ bool CONFIG::find(const char *needle, st
 const char *CONFIG::find(const char *needle, string_map &haystack) {
 	string_map::iterator i = haystack.find(needle);
 	if (i != haystack.end()) return (*i).second;		// found user@domain.tld key
-	char *x = strchr(needle, '@');
+	char *x = (char *) strchr(needle, '@');
 	if (x) {
 		x++;
 		i = haystack.find(x);
