travis-ci: disable GCC's -Wstringop-truncation noisy error on Ubuntu

This is the same as commit 4d86cae4f0 ("ci: disable GCC's
-Wstringop-truncation noisy error"), but for Ubuntu. Without this,
there are false positives in bpf_object__new() on Ubuntu 20.04:
this function calls strncpy() with the correct bounds, but still
triggers the warning.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>

travis-ci/managers/ubuntu.sh

@@ -14,7 +14,7 @@ source "$(dirname $0)/travis_wait.bash"
 
 cd $REPO_ROOT
 
-CFLAGS="-g -O2 -Werror -Wall -fsanitize=address,undefined"
+CFLAGS="-g -O2 -Werror -Wall -fsanitize=address,undefined -Wno-stringop-truncation"
 mkdir build install
 cc --version
 make -j$((4*$(nproc))) CFLAGS="${CFLAGS}" -C ./src -B OBJDIR=../build