bionic-amd64-tf-m-build: Update version of CppCheck

CppCheck v2.3 has a bug to report unexpected error of bad macro syntax.
This bug has been fixed in the latest version v2.6.
Update CppCheck to v2.6 on TF-M Open CI.

Signed-off-by: Xinyu Zhang <xinyu.zhang@arm.com>
Change-Id: Icd87d8c781b9879fa198a0b2f8f8aaae5c79f171
diff --git a/bionic-amd64-tf-m-build/tf-dependencies.install b/bionic-amd64-tf-m-build/tf-dependencies.install
index 7d9952a..dfc60e8 100755
--- a/bionic-amd64-tf-m-build/tf-dependencies.install
+++ b/bionic-amd64-tf-m-build/tf-dependencies.install
@@ -3,7 +3,7 @@
 set -e
 
 # Install cppcheck
-wget -q https://github.com/danmar/cppcheck/archive/2.3.tar.gz -O /tmp/cppcheck.tar.gz
+wget -q https://github.com/danmar/cppcheck/archive/2.6.tar.gz -O /tmp/cppcheck.tar.gz
 tar -xf /tmp/cppcheck.tar.gz -C /tmp
 (cd /tmp/cppcheck-*; make MATCHCOMPILER=yes FILESDIR=/usr/share/cppcheck -j$(nproc); make MATCHCOMPILER=yes FILESDIR=/usr/share/cppcheck install)