Added "const" to ngx_memcpy() with NGX_MEMCPY_LIMIT defined.

This fixes warning produced during compilation of the ngx_http_geoip_module
due to const qualifier being discarded.

src/core/ngx_string.c

diff --git a/src/core/ngx_string.c b/src/core/ngx_string.c
index 8aaa1d2..3b392f7 100644
--- a/src/core/ngx_string.c
+++ b/src/core/ngx_string.c
@@ -1827,7 +1827,7 @@
 #if (NGX_MEMCPY_LIMIT)
 
 void *
-ngx_memcpy(void *dst, void *src, size_t n)
+ngx_memcpy(void *dst, const void *src, size_t n)
 {
     if (n > NGX_MEMCPY_LIMIT) {
         ngx_log_error(NGX_LOG_ALERT, ngx_cycle->log, 0, "memcpy %uz bytes", n);

src/core/ngx_string.h

diff --git a/src/core/ngx_string.h b/src/core/ngx_string.h
index fa386ca..92d246e 100644
--- a/src/core/ngx_string.h
+++ b/src/core/ngx_string.h
@@ -89,7 +89,7 @@
 
 #if (NGX_MEMCPY_LIMIT)
 
-void *ngx_memcpy(void *dst, void *src, size_t n);
+void *ngx_memcpy(void *dst, const void *src, size_t n);
 #define ngx_cpymem(dst, src, n)   (((u_char *) ngx_memcpy(dst, src, n)) + (n))
 
 #else