pager: include stdint.h because uintmax_t is used

pager.h uses uintmax_t but does not include stdint.h. Therefore, add
this include statement.

This was discovered when writing a stub pager.c file.

Signed-off-by: Jonathan Tan <[email protected]>

pager.h

@@ -1,6 +1,8 @@
 #ifndef PAGER_H
 #define PAGER_H
 
+#include <stdint.h>
+
 struct child_process;
 
 const char *git_pager(int stdout_is_tty);