gdb: Fix style for Linuxbrew
gdb: Fix style for Linuxbrew

Convert if nested inside else to elsif.

LinuxbrewTestBot/homebrew-core,maxim-belkin/homebrew-core,rwhogg/homebrew-core,maxim-belkin/homebrew-core,rwhogg/homebrew-core,Linuxbrew/homebrew-core,Linuxbrew/homebrew-core,LinuxbrewTestBot/homebrew-core