tree-wide: fix the string concatenation warning with clang-12

tree-wide: fix the string concatenation warning with clang-12

e.g.:
./src/shared/dissect-image.c:2218:39: error: suspicious concatenation of string literals in an array initialization; did you mean to separate the elements with a comma? [-Werror,-Wstring-concatenation]
                                      "/usr/lib/os-release\0",
                                      ^
../src/shared/dissect-image.c:2217:39: note: place parentheses around the string literal to silence warning
                [META_OS_RELEASE]   = "/etc/os-release\0"
                                      ^
1 error generated.

See: https://reviews.llvm.org/D85545