[gnome-applets] build: add AC_HEADER_TIME to fix warnings

[Alberts Muktupāvels <muktupavels src gnome org>]

commit c4719e4ce924ac48ecb5fc02bce7589770acf86b
Author: Alberts Muktupāvels <alberts muktupavels gmail com>
Date:   Thu Apr 2 01:30:21 2020 +0300

    build: add AC_HEADER_TIME to fix warnings
    
    warning: "TIME_WITH_SYS_TIME" is not defined, evaluates to 0 [-Wundef]
    warning: "HAVE_SYS_TIME_H" is not defined, evaluates to 0 [-Wundef]

 configure.ac | 6 ++++++
 1 file changed, 6 insertions(+)
---
diff --git a/configure.ac b/configure.ac
index 2942e8c88..6e7cb65ca 100644
--- a/configure.ac
+++ b/configure.ac
@@ -56,6 +56,12 @@ dnl **************************************************************************
 LT_INIT([dlopen disable-static])
 LT_LIB_M
 
+dnl **************************************************************************
+dnl libgtop uses TIME_WITH_SYS_TIME in global.h
+dnl **************************************************************************
+
+AC_HEADER_TIME
+
 dnl ***************************************************************************
 dnl *** Minimum library versions for GNOME Applets                          ***
 dnl ***************************************************************************