pkgconfig: Add Requires.private field for static linking

[Biswa96]

Requires.private field is required to link dependent libraries while static linking. For example, here libjpeg and libheif. pkgconf -libs -static command will show the dependent libraries. This helps to manage the dependent libraries automatically in build system without adding them explicitly while static linking. More information about this feature can be found here https://people.freedesktop.org/~dbn/pkg-config-guide.html

[mdadams]

@jubalh This change seems reasonable to me, but I don't know much about pkgconfig. What do you think? @Biswa96 This is a minor quibble, but would you mind changing the "JAS_PC_REQUIRES" to either "JAS_PKGCONFIG_REQUIRES" as this would be a slightly more descriptive name?

[jubalh]

@mdadams I agree, it looks good.

[jubalh]

Thanks @Biswa96