build: rename directory "include" to "shared"

Up to now, the "include" directory contained (only) header files that were
used project-wide by libs, core, clients, et al.

Since the directory now also contains a non-header file, the "include"
name is misleading. Instead of adding yet another directory that is
project-wide, with non-header-only content, rename the "include"
directory to "shared".

src/dhcp-manager/Makefile.am

@@ -4,8 +4,8 @@ nm_dhcp_helper_SOURCES = nm-dhcp-helper.c
 
 nm_dhcp_helper_CPPFLAGS = \
 	$(GLIB_CFLAGS) \
-	-I$(top_srcdir)/include \
-	-I${top_builddir}/include \
+	-I$(top_srcdir)/shared \
+	-I${top_builddir}/shared \
 	-I$(top_srcdir)/libnm-core \
 	-I$(top_builddir)/libnm-core \
 	-DG_LOG_DOMAIN=\""nm-dhcp-helper"\" \

src/dhcp-manager/tests/Makefile.am

@@ -1,6 +1,6 @@
 AM_CPPFLAGS = \
-	-I$(top_srcdir)/include \
-	-I${top_builddir}/include \
+	-I$(top_srcdir)/shared \
+	-I${top_builddir}/shared \
 	-I${top_srcdir}/libnm-core \
 	-I${top_builddir}/libnm-core \
 	-I$(top_srcdir)/src/dhcp-manager \