Fix clang 8 string-plus-int warnings

Clang 8 warns that adding a string to an integer does not append to
string. Indeed it doesn't, but that was not the intentetion. Use array
indexing as suggested by the compiler to silence the warning. There
should be no functional change.

Example of warning message:

    alpm.c:71:54: warning: adding 'int' to a string does not append to the string [-Wstring-plus-int]
            sprintf(hookdir, "%s%s", myhandle->root, SYSHOOKDIR + 1);
                                                     ~~~~~~~~~~~^~~
    alpm.c:71:54: note: use array indexing to silence this warning
            sprintf(hookdir, "%s%s", myhandle->root, SYSHOOKDIR + 1);
                                                                ^
                                                     &          [  ]
    1 warning generated.

1 files changed, 1 insertions, 1 deletions

diff --git a/lib/libalpm/alpm.c b/lib/libalpm/alpm.c
index 0f3db6f4..bd7a129a 100644
--- a/lib/libalpm/alpm.c
+++ b/lib/libalpm/alpm.c
@@ -68,7 +68,7 @@ alpm_handle_t SYMEXPORT *alpm_initialize(const char *root, const char *dbpath,
 	 * with a slash) correctly, we skip SYSHOOKDIR[0]; the regular +1 therefore
 	 * disappears from the allocation */
 	MALLOC(hookdir, strlen(myhandle->root) + strlen(SYSHOOKDIR), goto nomem);
-	sprintf(hookdir, "%s%s", myhandle->root, SYSHOOKDIR + 1);
+	sprintf(hookdir, "%s%s", myhandle->root, &SYSHOOKDIR[1]);
 	myhandle->hookdirs = alpm_list_add(NULL, hookdir);
 
 	/* set default database extension */