Adding an Nvidia representative to security group

Adding Nikhil Gupta as an Nvidia representative to the llvm security
group.

Differential Revision: https://reviews.llvm.org/D106906

llvm/docs/Security.rst

@@ -41,6 +41,7 @@ The members of the group represent a wide cross-section of the community, and me
 * Kate McInnes (Apple) []
 * Kristof Beyls (ARM) [kristof.beyls]
 * Matthew Riley (Google) [mattdr]
+* Nikhil Gupta (Nvidia) [nikhgupt]
 * Oliver Hunt (Apple) [ojhunt]
 * Paul Robinson (Sony) [probinson]
 * Peter Smith (ARM) [peter.smith]