at: fix build issue with parallel builds

With highly parallel builds, the input files for parsetime.l are not
alway present when needed, breaking the build.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/at/at.mk

@@ -6,6 +6,8 @@
 AT_VERSION = 3.1.12
 AT_SOURCE = at_$(AT_VERSION).orig.tar.gz
 AT_SITE = $(BR2_DEBIAN_MIRROR)/debian/pool/main/a/at
+# missing deps for parsetime.l
+AT_MAKE = $(MAKE1)
 AT_AUTORECONF = YES
 AT_INSTALL_STAGING = NO
 AT_INSTALL_TARGET = YES