php: disable valgrind

Introduced in PHP7.2, if a host has valgrind headers installed, PHP will detect
them and set HAVE_VALGRIND to 1.
Disable this entry after configuring.

fixes:
http://autobuild.buildroot.net/results/d59/d59b5961890aeddcd6d59ed52243be6554d1fe21



Signed-off-by: Adam Duskett <aduskett@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>