build race with drmemfuncs vs event_strings

The cross-arch tests linux.execve{32,64} are failing on CDash. E.g.: http://dynamorio.org/CDash/testDetails.php?test=205911&build=41693

Looking more closely it's a build race b/c the newly added memmove.c to drmemfuncs did not come with an explicit dep on event_strings:

Scanning dependencies of target drmemfuncs
[  0%] Building ASM object core/CMakeFiles/drmemfuncs.dir/arch/x86/memfuncs.asm.o
[  0%] Building C object core/CMakeFiles/drmemfuncs.dir/lib/memmove.c.o
In file included from /work/dr/nightly/src/core/lib/../globals.h:345:0,
                 from /work/dr/nightly/src/core/lib/memmove.c:44:
/work/dr/nightly/src/core/lib/../utils.h:1944:10: fatal error: event_strings.h: No such file or directory
 #include "event_strings.h"
          ^~~~~~~~~~~~~~~~~