completion: Add shellcheck shell hints to all completion scripts

Because completion scripts are not executed directly, they don’t have a
shebang line, so shellcheck can’t be sure which shell syntax to use for
them. Help it out.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

gio/completion/gapplication

@@ -1,4 +1,6 @@
 
+# shellcheck shell=bash
+
 # Check for bash
 [ -z "$BASH_VERSION" ] && return
 

gio/completion/gdbus

@@ -1,4 +1,6 @@
 
+# shellcheck shell=bash
+
 # Check for bash
 [ -z "$BASH_VERSION" ] && return
 

gio/completion/gio

@@ -15,6 +15,8 @@
 # along with this library; if not, see <http://www.gnu.org/licenses/>.
 #
 
+# shellcheck shell=bash
+
 # Check for bash
 [ -z "$BASH_VERSION" ] && return
 

gio/completion/gresource

@@ -1,4 +1,6 @@
 
+# shellcheck shell=bash
+
 # Check for bash
 [ -z "$BASH_VERSION" ] && return
 

gio/completion/gsettings

@@ -1,4 +1,6 @@
 
+# shellcheck shell=bash
+
 # Check for bash
 [ -z "$BASH_VERSION" ] && return