Fix compiling --without-libcurl.

Previously <curl/curl.h> was included even if compiled --without-libcurl.
This can fail when libcurl headers are not there. This commit guards this
include by checks for HAVE_LIBCURL.

src/backend/distributed/utils/statistics_collection.c

@@ -10,6 +10,10 @@
 
 #include "postgres.h"
 
+bool EnableStatisticsCollection = true; /* send basic usage statistics to Citus */
+
+#if HAVE_LIBCURL
+
 #include <curl/curl.h>
 #include <sys/utsname.h>
 
@@ -21,10 +25,6 @@
 #include "lib/stringinfo.h"
 #include "utils/json.h"
 
-bool EnableStatisticsCollection = true; /* send basic usage statistics to Citus */
-
-#if HAVE_LIBCURL
-
 static uint64 NextPow2(uint64 n);
 static uint64 ClusterSize(List *distributedTableList);
 static bool SendHttpPostJsonRequest(const char *url, const char *postFields, long