Add missing #include.

Fixes https://github.com/boostorg/regex/issues/183

src/posix_api.cpp

@@ -21,6 +21,7 @@
 #include <boost/config.hpp>
 #include <boost/regex.hpp>
 #include <boost/cregex.hpp>
+#include <boost/cstdint.hpp>
 #include <cstdio>
 
 #if defined(BOOST_NO_STDC_NAMESPACE)

src/wide_posix_api.cpp

@@ -24,6 +24,7 @@
 
 #include <boost/regex.hpp>
 #include <boost/cregex.hpp>
+#include <boost/cstdint.hpp>
 
 #include <cstdio>
 #include <cstring>