Avoid warning about alloca in read.c

[Eli Zaretskii]

       CC       libguile_2.0_la-read.lo
     read.c: In function 'try_read_ci_chars':
     read.c:983:3: warning: implicit declaration of function 'alloca' 
[-Wimplicit-function-declaration]
     read.c:983:22: warning: incompatible implicit declaration of built-in 
function 'alloca' [enabled by default]

The patch to avoid this warning is below.  OK to commit?

--- libguile/read.c~0   2016-01-02 16:24:55.000000000 +0200
+++ libguile/read.c     2016-07-15 12:38:42.195125000 +0300
@@ -33,6 +33,10 @@
 #include <c-strcase.h>
 #include <c-ctype.h>
 
+#ifdef __MINGW32__
+#include <alloca.h>
+#endif
+
 #include "libguile/_scm.h"
 #include "libguile/bytevectors.h"
 #include "libguile/chars.h"